ext3: Fix error handling on inode bitmap corruption

When insert_inode_locked() fails in ext3_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 inode does not have I_NEW set. Fix the problem by jumping
to fail: (instead of fail_drop:) which declares filesystem error and
does not call unlock_new_inode().