zfs_vnops_os.c: Add support for the _PC_CLONE_BLKSIZE name

FreeBSD now has a pathconf name called _PC_CLONE_BLKSIZE
which is the block size supported for block cloning for
the file system. Since ZFS's block size varies per file,
return the largest size likely to be used, or zero if block
cloning is not supported.

Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Rick Macklem <rmacklem@uoguelph.ca>
Closes #17645