libext2fs/e2fsck: refactor everyone who writes zero blocks to disk

libext2fs/e2fsck: refactor everyone who writes zero blocks to disk

Convert all call sites that write zero blocks to disk to use
ext2fs_zero_blocks2() since it can use Linux's zero out feature to do
the writes more quickly.  Reclaim the zero buffer at freefs time and
make the write-zeroes fallback use a larger buffer.

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