Btrfs: expand free space tree sanity tests to catch endianness bug

The free space tree format conversion functions were broken on
big-endian systems, but the sanity tests didn't catch it because all of
the operations were aligned to multiple words. This was meant to catch
any bugs in the extent buffer code's handling of high memory, but it
ended up hiding the endianness bug. Expand the tests to do both
sector-aligned and page-aligned operations.

Tested-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>