btrfs: make alloc_extent_buffer() check subpage dirty bitmap

In alloc_extent_buffer(), we make sure that the newly allocated page is
never dirty.

This is fine for sector size == PAGE_SIZE case, but for subpage it's
possible that one extent buffer in the page is dirty, thus the whole
page is marked dirty, and could cause false alert.

To support subpage, call btrfs_page_test_dirty() to handle both cases.

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