MAINTAINERS: associate linux/fs.h with VFS instead of file locking

include/linux/fs.h and include/uapi/linux/fs.h deal with much more than
just file locking.  Move them to the "FILESYSTEMS (VFS and
infrastructure)" section of the MAINTAINERS file so that the first
suggestion from get_maintainer.pl isn't the file locking maintainers,
which has caused some confusion.

Cc: J. Bruce Fields <bfields@fieldses.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>