ext2's inode allocator will call find_group_orlov(), which will return a
suitable blockgroup in which the inode should be allocated.  But by the time
we actually try to allocate an inode in the blockgroup, other CPUs could have
used them all up.

ext2 will bogusly fail with "ext2_new_inode: Free inodes count corrupted in
group NN".


To fix this we just advance onto the next blockgroup if the rare race
happens.  If we've scanned all blockgroups then return -ENOSPC.


(This is a bit inaccurate: after we've scanned all blockgroups, there may
still be available inodes due to inode freeing activity in other blockgroups.
 This cannot be fixed without fs-wide locking.  The effect is a slightly
early ENOSPC in a nearly-full filesystem).