e2fsck: Try to update on-disk bitmap in e2fsck_get_alloc_block() callback

e2fsck: Try to update on-disk bitmap in e2fsck_get_alloc_block() callback

The e2fsck_get_alloc_block() callback is used so that if the ext2fs
library needs to allocate blocks internally (most notably by the
extents functions), e2fsck's internal block usage map is consulted
since it is the only thing that can be trusted during a large part of
e2fsck's operation.

Change it to update the on-disk bitmap if it is loaded.  This reduces
the number of spurious differences found in pass #5.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>