e2fsck: fix sliding the directory block down on bigalloc

e2fsck: fix sliding the directory block down on bigalloc

If we find a hole in a directory on a bigalloc filesystem, we need to
obey the cluster alignment rules when collapsing the gap to avoid
later complaints.

Specifically, the calculation of the new logical cluster number was
incorrect, and we need to ensure that the logical cluster alignment
respects the physical cluster alignment, since we've concluded that
the extent's logical block number is wrong.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>