btrfs: comment and minor simplifications in run_delalloc_nocow

Add a comment explaining why we keep the BUG also use the already read
and cached value of extent ram bytes stored in 'ram_bytes'.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -1503,16 +1503,15 @@ static noinline int run_delalloc_nocow(struct inode *inode,
 				goto out_check;
 			nocow = true;
 		} else if (extent_type == BTRFS_FILE_EXTENT_INLINE) {
-			extent_end = found_key.offset +
-				btrfs_file_extent_ram_bytes(leaf, fi);
-			extent_end = ALIGN(extent_end,
-					   fs_info->sectorsize);
+			extent_end = found_key.offset + ram_bytes;
+			extent_end = ALIGN(extent_end, fs_info->sectorsize);
 			/* Skip extents outside of our requested range */
 			if (extent_end <= start) {
 				path->slots[0]++;
 				goto next_slot;
 			}
 		} else {
+			/* If this triggers then we have a memory corruption */
 			BUG();
 		}
 out_check: