btrfs: raid56: remove sector_ptr::has_paddr member

btrfs: raid56: remove sector_ptr::has_paddr member

We can use paddr -1 as an indicator for unset/uninitialized paddr.

We can not use 0 paddr, unlike virtual address 0 which is never mapped
thus will always trigger a page fault, physical address 0 may be a valid
page.

So here we follow swiotlb to use (paddr)-1 as a special indicator for
invalid/unset physical address.

Even if the PFN may still be valid, our usage of the physical address
should always be aligned to fs block size (or page size for bs > ps
cases), thus such -1 paddr should never be a valid one.

With this special -1 paddr, we can get rid of has_paddr member and save
1 byte for sector_ptr structure.

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