btrfs: drop redundant bflags initialization when allocating extent buffer

When allocating an extent buffer, at __alloc_extent_buffer(), there's no
point in explicitly assigning zero to the bflags field of the new extent
buffer because we allocated it with kmem_cache_zalloc().

So just remove the redundant initialization, it saves one mov instruction
in the generated assembly code for x86_64 ("movq $0x0,0x10(%rax)").
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -4266,7 +4266,6 @@ __alloc_extent_buffer(struct btrfs_fs_info *fs_info, u64 start,
 	eb->start = start;
 	eb->len = len;
 	eb->fs_info = fs_info;
-	eb->bflags = 0;
 	init_rwsem(&eb->lock);
 
 	btrfs_leak_debug_add_eb(eb);