gfs2: Add gfs2_max_stuffed_size

Add a small inline function for computing the maximum size of a stuffed
inode instead of open coding that in several places throughout the code.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
diff --git a/fs/gfs2/dir.c b/fs/gfs2/dir.c
index 06a0d19..7c21aea 100644
--- a/fs/gfs2/dir.c
+++ b/fs/gfs2/dir.c
@@ -170,8 +170,7 @@ static int gfs2_dir_write_data(struct gfs2_inode *ip, const char *buf,
 	if (!size)
 		return 0;
 
-	if (gfs2_is_stuffed(ip) &&
-	    offset + size <= sdp->sd_sb.sb_bsize - sizeof(struct gfs2_dinode))
+	if (gfs2_is_stuffed(ip) && offset + size <= gfs2_max_stuffed_size(ip))
 		return gfs2_dir_write_stuffed(ip, buf, (unsigned int)offset,
 					      size);