libext2fs: ext2fs_new_block2() should call alloc_block hook

libext2fs: ext2fs_new_block2() should call alloc_block hook

If ext2fs_new_block2() is called without a specific block map, we
should call the alloc_block hook before checking fs->block_map.  This
helps us to avoid a bug in e2fsck where we need to allocate a block
but instead of consulting block_found_map, we use the FS bitmaps,
which (prior to pass 5) could be wrong.

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