btrfs: add sysfs interface for supported sectorsize

Export supported sector sizes in /sys/fs/btrfs/features/supported_sectorsizes.

Currently all architectures have PAGE_SIZE, There's some disparity
between read-only and read-write support but that will be unified in the
future so there's only one file exporting the size.

The read-only support for systems with 64K pages also works for 4K
sector size.

This new sysfs interface would help eg. mkfs.btrfs to print more
accurate warnings about potentially incompatible option combinations.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>