btrfs: new define for the inline extent data start

Use a common definition for the inline data start so we don't have to
open-code it and introduce bugs like "Btrfs: fix wrong max inline data
size limit" fixed.
Signed-off-by: David Sterba <dsterba@suse.cz>

fs/btrfs/ctree.c

@@ -4625,8 +4625,7 @@ void btrfs_truncate_item(struct btrfs_root *root, struct btrfs_path *path,
 				ptr = btrfs_item_ptr_offset(leaf, slot);
 				memmove_extent_buffer(leaf, ptr,
 				      (unsigned long)fi,
-				      offsetof(struct btrfs_file_extent_item,
-						 disk_bytenr));
+				      BTRFS_FILE_EXTENT_INLINE_DATA_START);
 			}
 		}
 

fs/btrfs/ctree.h

@@ -385,9 +385,11 @@ struct btrfs_header {
 				     sizeof(struct btrfs_key_ptr))
 #define __BTRFS_LEAF_DATA_SIZE(bs) ((bs) - sizeof(struct btrfs_header))
 #define BTRFS_LEAF_DATA_SIZE(r) (__BTRFS_LEAF_DATA_SIZE(r->nodesize))
+#define BTRFS_FILE_EXTENT_INLINE_DATA_START		\
+		(offsetof(struct btrfs_file_extent_item, disk_bytenr))
 #define BTRFS_MAX_INLINE_DATA_SIZE(r) (BTRFS_LEAF_DATA_SIZE(r) - \
 					sizeof(struct btrfs_item) - \
-					offsetof(struct btrfs_file_extent_item, disk_bytenr))
+					BTRFS_FILE_EXTENT_INLINE_DATA_START)
 #define BTRFS_MAX_XATTR_SIZE(r)	(BTRFS_LEAF_DATA_SIZE(r) - \
 				 sizeof(struct btrfs_item) -\
 				 sizeof(struct btrfs_dir_item))
@@ -896,6 +898,8 @@ struct btrfs_file_extent_item {
 	/*
 	 * disk space consumed by the extent, checksum blocks are included
 	 * in these numbers
+	 *
+	 * At this offset in the structure, the inline extent data start.
 	 */
 	__le64 disk_bytenr;
 	__le64 disk_num_bytes;
@@ -3043,14 +3047,12 @@ BTRFS_SETGET_STACK_FUNCS(stack_file_extent_compression,
 static inline unsigned long
 btrfs_file_extent_inline_start(struct btrfs_file_extent_item *e)
 {
-	unsigned long offset = (unsigned long)e;
-	offset += offsetof(struct btrfs_file_extent_item, disk_bytenr);
-	return offset;
+	return (unsigned long)e + BTRFS_FILE_EXTENT_INLINE_DATA_START;
 }
 
 static inline u32 btrfs_file_extent_calc_inline_size(u32 datasize)
 {
-	return offsetof(struct btrfs_file_extent_item, disk_bytenr) + datasize;
+	return BTRFS_FILE_EXTENT_INLINE_DATA_START + datasize;
 }
 
 BTRFS_SETGET_FUNCS(file_extent_disk_bytenr, struct btrfs_file_extent_item,
@@ -3080,9 +3082,7 @@ BTRFS_SETGET_FUNCS(file_extent_other_encoding, struct btrfs_file_extent_item,
 static inline u32 btrfs_file_extent_inline_item_len(struct extent_buffer *eb,
 						    struct btrfs_item *e)
 {
-	unsigned long offset;
-	offset = offsetof(struct btrfs_file_extent_item, disk_bytenr);
-	return btrfs_item_size(eb, e) - offset;
+	return btrfs_item_size(eb, e) - BTRFS_FILE_EXTENT_INLINE_DATA_START;
 }
 
 /* this returns the number of file bytes represented by the inline item.