Consolidate some fiddly code for changing an inode's snap_realm
into a new helper function, and change the callers to use it.

While we're in here, nothing uses the i_snap_realm_counter field, so
remove that from the inode.
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Luis Henriques <lhenriques@suse.de>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>