nfsd: trivial: use SEEK_SET instead of 0 in vfs_llseek

They're equivalent, but SEEK_SET is more informative...

Cc: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>