gfs2: Zero out fallocated blocks in fallocate_chunk

Instead of zeroing out fallocated blocks in gfs2_iomap_alloc, zero them
out in fallocate_chunk, much higher up the call stack.  This gets rid of
gfs2's abuse of the IOMAP_ZERO flag as well as the gfs2 specific zeronew
buffer flag.  I can't think of a reason why zeroing out the blocks in
gfs2_iomap_alloc would have any benefits: there is no additional locking
at that level that would add protection to the newly allocated blocks.

While at it, change fallocate over from gs2_block_map to gfs2_iomap_begin.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Acked-by: Christoph Hellwig <hch@lst.de>