include/linux/fs.h: remove unneeded forward definition of mm_struct

Link: http://lkml.kernel.org/r/20170525102927.6163-1-jlayton@redhat.com


Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>