btrfs: simplify support block size check

Currently we manually check the block size against 3 different values:

- 4K
- PAGE_SIZE
- MIN_BLOCKSIZE

Those 3 values can match or differ from each other.  This makes it
pretty complex to output the supported block sizes.

Considering we're going to add block size > page size support soon, this
can make the support block size sysfs attribute much harder to
implement.

To make it easier, factor out a helper, btrfs_supported_blocksize() to
do a simple check for the block size.

Then utilize it in the two locations:

- btrfs_validate_super()
  This is very straightforward

- supported_sectorsizes_show()
  Iterate through all valid block sizes, and only output supported ones.

  This is to make future full range block sizes support much easier.

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>