GFS2: rewrite fallocate code to write blocks directly

GFS2's fallocate code currently goes through the page cache. Since it's only
writing to the end of the file or to holes in it, it doesn't need to, and it
was causing issues on low memory environments. This patch pulls in some of
Steve's block allocation work, and uses it to simply allocate the blocks for
the file, and zero them out at allocation time.  It provides a slight
performance increase, and it dramatically simplifies the code.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
static int trunc_start(struct inode *inode, u64 oldsize, u64 newsize)
{
	struct gfs2_inode *ip = GFS2_I(inode);
	struct gfs2_sbd *sdp = GFS2_SB(inode);
	struct address_space *mapping = inode->i_mapping;
	struct buffer_head *dibh;
	int journaled = gfs2_is_jdata(ip);
	int error;

	error = gfs2_trans_begin(sdp,
				 RES_DINODE + (journaled ? RES_JDATA : 0), 0);
	if (error)
		return error;

	error = gfs2_meta_inode_buffer(ip, &dibh);
	if (error)
		goto out;

	gfs2_trans_add_bh(ip->i_gl, dibh, 1);

	if (gfs2_is_stuffed(ip)) {
		gfs2_buffer_clear_tail(dibh, sizeof(struct gfs2_dinode) + newsize);
	} else {
		if (newsize & (u64)(sdp->sd_sb.sb_bsize - 1)) {
			error = gfs2_block_truncate_page(mapping, newsize);
			if (error)
				goto out_brelse;
		}
		ip->i_diskflags |= GFS2_DIF_TRUNC_IN_PROG;
	}

	i_size_write(inode, newsize);
	ip->i_inode.i_mtime = ip->i_inode.i_ctime = CURRENT_TIME;
	gfs2_dinode_out(ip, dibh->b_data);

	truncate_pagecache(inode, oldsize, newsize);
out_brelse:
	brelse(dibh);
out:
	gfs2_trans_end(sdp);
	return error;
}

static int trunc_start(struct inode *inode, u64 oldsize, u64 newsize)
{
	struct gfs2_inode *ip = GFS2_I(inode);
	struct gfs2_sbd *sdp = GFS2_SB(inode);
	struct address_space *mapping = inode->i_mapping;
	struct buffer_head *dibh;
	int journaled = gfs2_is_jdata(ip);
	int error;

	error = gfs2_trans_begin(sdp,
				 RES_DINODE + (journaled ? RES_JDATA : 0), 0);
	if (error)
		return error;

	error = gfs2_meta_inode_buffer(ip, &dibh);
	if (error)
		goto out;

	gfs2_trans_add_bh(ip->i_gl, dibh, 1);

	if (gfs2_is_stuffed(ip)) {
		gfs2_buffer_clear_tail(dibh, sizeof(struct gfs2_dinode) + newsize);
	} else {
		if (newsize & (u64)(sdp->sd_sb.sb_bsize - 1)) {
			error = gfs2_block_truncate_page(mapping, newsize);
			if (error)
				goto out_brelse;
		}
		ip->i_diskflags |= GFS2_DIF_TRUNC_IN_PROG;
	}

	i_size_write(inode, newsize);
	ip->i_inode.i_mtime = ip->i_inode.i_ctime = CURRENT_TIME;
	gfs2_dinode_out(ip, dibh->b_data);

	truncate_pagecache(inode, oldsize, newsize);
out_brelse:
	brelse(dibh);
out:
	gfs2_trans_end(sdp);
	return error;
}
