xfs: return from xfs_symlink_verify early on V4 filesystems

V4 symlink blocks didn't have headers, so return early if this is a V4
filesystem.

Cc: <stable@vger.kernel.org> # v5.1
Fixes: 39708c20 ("xfs: miscellaneous verifier magic value fixups")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>