Btrfs: fix extent buffer bitmap tests on big-endian systems

The in-memory bitmap code manipulates words and is therefore sensitive
to endianness, while the extent buffer bitmap code addresses bytes and
is byte-order agnostic. Because the byte addressing of the extent buffer
bitmaps is equivalent to a little-endian in-memory bitmap, the extent
buffer bitmap tests fail on big-endian systems.

34b3e6c9 ("Btrfs: self-tests: Fix extent buffer bitmap test fail on
BE system") worked around another endianness bug in the tests but missed
this one because ed9e4afd ("Btrfs: self-tests: Execute page
straddling test only when nodesize < PAGE_SIZE") disables this part of
the test on ppc64. That change lost the original meaning of the test,
however. We really want to test that an equivalent series of operations
using the in-memory bitmap API and the extent buffer bitmap API produces
equivalent results.

To fix this, don't use memcmp_extent_buffer() or write_extent_buffer();
do everything bit-by-bit.

Reported-by: Anatoly Pugachev <matorola@gmail.com>
Tested-by: Anatoly Pugachev <matorola@gmail.com>
Tested-by: Feifei Xu <xufeifei@linux.vnet.ibm.com>
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>