btrfs: simplify eb checksum verification in btrfs_validate_metadata_buffer

The verification copies the calculated checksum bytes to a temporary
buffer but this is not necessary. We can map the eb header on the first
page and use the checksum bytes directly.

This saves at least one function call and boundary checks so it could
lead to a minor performance improvement.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -584,6 +584,7 @@ static int validate_extent_buffer(struct extent_buffer *eb)
 	const u32 csum_size = fs_info->csum_size;
 	u8 found_level;
 	u8 result[BTRFS_CSUM_SIZE];
+	const u8 *header_csum;
 	int ret = 0;
 
 	found_start = btrfs_header_bytenr(eb);
@@ -608,15 +609,14 @@ static int validate_extent_buffer(struct extent_buffer *eb)
 	}
 
 	csum_tree_block(eb, result);
+	header_csum = page_address(eb->pages[0]) +
+		get_eb_offset_in_page(eb, offsetof(struct btrfs_header, csum));
 
-	if (memcmp_extent_buffer(eb, result, 0, csum_size)) {
-		u8 val[BTRFS_CSUM_SIZE] = { 0 };
-
-		read_extent_buffer(eb, &val, 0, csum_size);
+	if (memcmp(result, header_csum, csum_size) != 0) {
 		btrfs_warn_rl(fs_info,
 	"checksum verify failed on %llu wanted " CSUM_FMT " found " CSUM_FMT " level %d",
 			      eb->start,
-			      CSUM_FMT_VALUE(csum_size, val),
+			      CSUM_FMT_VALUE(csum_size, header_csum),
 			      CSUM_FMT_VALUE(csum_size, result),
 			      btrfs_header_level(eb));
 		ret = -EUCLEAN;