ext2: Fix error handling on inode bitmap corruption

When insert_inode_locked() fails in ext2_new_inode() it most likely means inode
bitmap got corrupted and we allocated again inode which is already in use. Also
doing unlock_new_inode() during error recovery is wrong since the inode does
not have I_NEW set. Fix the problem by informing about filesystem error and
jumping to fail: (instead of fail_drop:) which doesn't call unlock_new_inode().

Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>