btrfs: subpage: make reader lock utilize bitmap

Currently btrfs_subpage utilizes its atomic member @reader to manage the
reader counter.  However it is only utilized to prevent the page to be
released/unlocked when we still have reads underway.

In that use case, we don't really allow multiple readers on the same
subpage sector.  So here we can introduce a new locked bitmap to
represent exactly which subpage range is locked for read.

In theory we can remove btrfs_subpage::reader as it's just the set bits
of the new locked bitmap.  But unfortunately bitmap doesn't provide such
handy API yet, so we still keep the reader counter.

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