Fix logging in l2arc_rebuild()

[gamanakis]

Motivation and Context

In case the L2ARC rebuild was canceled, do not log to spa history
log as the pool may be in the process of being removed and a panic
may occur:

BUG: kernel NULL pointer dereference, address: 0000000000000018
RIP: 0010:spa_history_log_internal+0xb1/0x120 [zfs]
Call Trace:
l2arc_rebuild+0x464/0x7c0 [zfs]
l2arc_dev_rebuild_start+0x2d/0x130 [zfs]
? l2arc_rebuild+0x7c0/0x7c0 [zfs]
thread_generic_wrapper+0x78/0xb0 [spl]
kthread+0xfb/0x130
? IS_ERR+0x10/0x10 [spl]
? kthread_park+0x90/0x90
ret_from_fork+0x35/0x40

Description

This may happen when the L2ARC device is very large resulting in a long rebuild L2ARC time, so that the user can actually issue a pool export.

How Has This Been Tested?

The bug can be triggered easily by putting a
delay(SEC_TO_TICK(1)); in the main for loop of l2arc_rebuild() and exporting the pool during L2ARC rebuild.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov]

Codecov Report

Merging #10659 into master will increase coverage by 0.13%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #10659      +/-   ##
==========================================
+ Coverage   79.55%   79.68%   +0.13%     
==========================================
  Files         394      394              
  Lines      124631   124633       +2     
==========================================
+ Hits        99146    99316     +170     
+ Misses      25485    25317     -168     

Flag	Coverage Δ
#kernel	80.20% <0.00%> (+0.07%)	⬆️
#user	65.79% <0.00%> (+0.87%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
module/zfs/arc.c	89.10% <0.00%> (+<0.01%)	⬆️
cmd/zdb/zdb_il.c	30.86% <0.00%> (-24.08%)	⬇️
module/zfs/vdev_indirect.c	72.66% <0.00%> (-12.34%)	⬇️
lib/libspl/os/linux/getmntany.c	71.73% <0.00%> (-6.53%)	⬇️
module/zfs/vdev_indirect_mapping.c	93.23% <0.00%> (-5.80%)	⬇️
lib/libzfs/libzfs_changelist.c	85.01% <0.00%> (-1.13%)	⬇️
module/zfs/spa_log_spacemap.c	93.40% <0.00%> (-0.86%)	⬇️
cmd/zed/agents/zfs_mod.c	77.55% <0.00%> (-0.67%)	⬇️
module/zfs/zfs_fm.c	84.39% <0.00%> (-0.58%)	⬇️
module/os/linux/zfs/zpl_xattr.c	83.26% <0.00%> (-0.43%)	⬇️
... and 59 more

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