btrfs: grab fs_info from extent_buffer in btrfs_mark_buffer_dirty

Since commit f28491e0 ("Btrfs: move the extent buffer radix tree into
the fs_info"), fs_info can be grabbed from extent_buffer directly.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>