block_cloning: multiple bugfixes on FreeBSD

[mmatuska]

Motivation and Context

FreeBSD:

• if copying files on the same dataset and block_cloning is not enabled, the copy is refused and EXDEV is returned
• zfs_freebsd_copy_file_range() may panic if

Description

Fall back to vn_generic_copy_file_range() if EXDEV is returned.
Use a_outcred if a_fsizetd is NULL

How Has This Been Tested?

Local testing

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)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS 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.

[mcmilk]

Does not build on my FreeBSD 13.1 box:

lk/src/zfs.mmatuska/module/os/freebsd/zfs/zfs_vnops_os.c -o zfs_vnops_os.o
/home/mcmilk/src/zfs.mmatuska/module/os/freebsd/zfs/zfs_vnops_os.c:6286:6: error: incompatible pointer types assigning to 'struct thread *' from 'struct ucred *' [-Werror,-Wincompatible-pointer-types]
                td = ap->a_outcred;
                   ^ ~~~~~~~~~~~~~
/home/mcmilk/src/zfs.mmatuska/module/os/freebsd/zfs/zfs_vnops_os.c:6288:6: error: incompatible pointer types assigning to 'struct thread *' from 'struct ucred *' [-Werror,-Wincompatible-pointer-types]
                td = ap->a_fsizetd->td_ucred;
                   ^ ~~~~~~~~~~~~~~~~~~~~~~~
/home/mcmilk/src/zfs.mmatuska/module/os/freebsd/zfs/zfs_vnops_os.c:6291:33: error: incompatible pointer types passing 'struct thread *' to parameter of type 'cred_t *' (aka 'struct ucred *') [-Werror,-Wincompatible-pointer-types]
            ap->a_outoffp, ap->a_lenp, td);
                                       ^~
/home/mcmilk/src/zfs.mmatuska/include/sys/zfs_vnops.h:35:25: note: passing argument to parameter here
    uint64_t *, cred_t *);
                        ^
3 errors generated.
*** Error code 1

[mmatuska]

Closed in favor of #14723