libext2fs: support allocating uninit blocks in bmap2()

libext2fs: support allocating uninit blocks in bmap2()

As part of supporting fallocate-like functionality, extend
ext2fs_bmap() with two flags -- BMAP_UNINIT and BMAP_ZERO.  The first
will cause it to mark/set a block uninitialized, if it's part of an
extent based file.  For a block mapped file, the mapping is put in,
but there is no way to remember the uninitialized status.  The second
flag causes the block to be zeroed to support the use case of
emulating uninitialized blocks on a block-map file by zeroing them.

Eventually fallocate or fuse2fs or somebody will use these.

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