FreeBSD: Reduce copy_file_range() source lock to shared

Linux locks copy_file_range() source as shared. FreeBSD was doing
it also, but then was changed to exclusive, partially because KPI
of that time was doing so, and partially seems out of caution.
Considering zfs_clone_range() uses range locks on both source and
destination, neither should require exclusive vnode locks. But one
step at a time, just sync it with Linux for now.

Reviewed-by: Alan Somers <asomers@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16789
Closes #16797