fs: btrfs: Reject fs with sector size other than PAGE_SIZE

Although in theory u-boot fs driver could easily support more sector
sizes, current code base doesn't have good enough way to grab sector
size yet.

This would cause problem for later LZO fixes which rely on sector size.

And considering that most u-boot boards are using 4K page size, which is
also the most common sector size for btrfs, rejecting fs with
non-page-sized sector size shouldn't cause much problem.

This should only be a quick fix before we implement better sector size
support.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Cc: Marek Behun <marek.behun@nic.cz>
Reviewed-by: Marek Behún <marek.behun@nic.cz>