minixfs: misplaced checks lead to dentry leak

bitmap size sanity checks should be done *before* allocating ->s_root;
there their cleanup on failure would be correct.  As it is, we do iput()
on root inode, but leak the root dentry...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>