xfs: Fix tail rounding in xfs_alloc_file_space()

To ensure that all blocks touched by the range [offset, offset + count)
are allocated, we need to calculate the block count from the difference
of the range end (rounded up) and the range start (rounded down).

Before this patch, we just round up the byte count, which may lead to
unaligned ranges not being fully allocated:

$ touch test_file
$ block_size=$(stat -fc '%S' test_file)
$ fallocate -o $((block_size / 2)) -l $block_size test_file
$ xfs_bmap test_file
test_file:
        0: [0..7]: 1396264..1396271
        1: [8..15]: hole

There should not be a hole there.  Instead, the first two blocks should
be fully allocated.

With this patch applied, the result is something like this:

$ touch test_file
$ block_size=$(stat -fc '%S' test_file)
$ fallocate -o $((block_size / 2)) -l $block_size test_file
$ xfs_bmap test_file
test_file:
        0: [0..15]: 11024..11039
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_bmap_util.c

@@ -864,6 +864,7 @@ xfs_alloc_file_space(
 	xfs_filblks_t		allocatesize_fsb;
 	xfs_extlen_t		extsz, temp;
 	xfs_fileoff_t		startoffset_fsb;
+	xfs_fileoff_t		endoffset_fsb;
 	int			nimaps;
 	int			quota_flag;
 	int			rt;
@@ -891,7 +892,8 @@ xfs_alloc_file_space(
 	imapp = &imaps[0];
 	nimaps = 1;
 	startoffset_fsb	= XFS_B_TO_FSBT(mp, offset);
-	allocatesize_fsb = XFS_B_TO_FSB(mp, count);
+	endoffset_fsb = XFS_B_TO_FSB(mp, offset + count);
+	allocatesize_fsb = endoffset_fsb - startoffset_fsb;
 
 	/*
 	 * Allocate file space until done or until there is an error