The bug time sequence:
1. context openzfs#1, `zfs_write` assign a txg "n".
2. In a same process, context openzfs#2, mmap page fault (which means the
   `mm_sem` is hold) occurred, `zfs_dirty_inode` open a txg failed,
   and wait previous txg "n" completed.
3. context openzfs#1 call `uiomove` to write, however page fault is occurred
   in `uiomove`, which means it need `mm_sem`, but `mm_sem` is hold by
   context openzfs#2, so it stuck and can't complete,  then txg "n" will
   not complete.

So context openzfs#1 and context openzfs#2 trap into the "dead lock".

Signed-off-by: Grady Wong <grady.w@xtaotech.com>

Signed-off-by: Grady Wong <grady.w@xtaotech.com>

Signed-off-by: Grady Wong <grady.w@xtaotech.com>

Signed-off-by: Grady Wong <grady.w@xtaotech.com>

Signed-off-by: Grady Wong <grady.w@xtaotech.com>

Signed-off-by: Grady Wong <grady.w@xtaotech.com>