xfs: Fix dax inode extent calculation when direct write is performed on an unwritten extent

With dax enabled filesystems, a direct write operation into an existing
unwritten extent results in xfs_iomap_write_direct() zero-ing and converting
the extent into a normal extent before the actual data is copied from the
userspace buffer.

The inode extent count can increase by 2 if the extent range being written to
maps to the middle of the existing unwritten extent range. Hence this commit
uses XFS_IEXT_WRITE_UNWRITTEN_CNT as the extent count delta when such a write
operation is being performed.

Fixes: 727e1acd ("xfs: Check for extent overflow when trivally adding a new extent")
Reported-by: NDarrick J. Wong <djwong@kernel.org>
Signed-off-by: NChandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: NDarrick J. Wong <djwong@kernel.org>
Signed-off-by: NDarrick J. Wong <djwong@kernel.org>
Reviewed-by: NChristoph Hellwig <hch@lst.de>

fs/xfs/xfs_iomap.c

@@ -198,6 +198,7 @@ xfs_iomap_write_direct(
 	bool			force = false;
 	int			error;
 	int			bmapi_flags = XFS_BMAPI_PREALLOC;
+	int			nr_exts = XFS_IEXT_ADD_NOSPLIT_CNT;
 
 	ASSERT(count_fsb > 0);
 
@@ -232,6 +233,7 @@ xfs_iomap_write_direct(
 		bmapi_flags = XFS_BMAPI_CONVERT | XFS_BMAPI_ZERO;
 		if (imap->br_state == XFS_EXT_UNWRITTEN) {
 			force = true;
+			nr_exts = XFS_IEXT_WRITE_UNWRITTEN_CNT;
 			dblocks = XFS_DIOSTRAT_SPACE_RES(mp, 0) << 1;
 		}
 	}
@@ -241,8 +243,7 @@ xfs_iomap_write_direct(
 	if (error)
 		return error;
 
-	error = xfs_iext_count_may_overflow(ip, XFS_DATA_FORK,
-			XFS_IEXT_ADD_NOSPLIT_CNT);
+	error = xfs_iext_count_may_overflow(ip, XFS_DATA_FORK, nr_exts);
 	if (error)
 		goto out_trans_cancel;