fuse: fix thinko in max I/O size calucation

Use max not min to enforce a lower limit on the max I/O size.

This bug was introduced by "fuse: fix max i/o size calculation" (commit
e5d9a0df).

Thanks to Brian Wang for noticing.

Reported-by: Brian Wang <ywang221@hotmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: Szabolcs Szakacsits <szaka@ntfs-3g.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>