e2image: fix metadata image handling on big endian systems

e2image: fix metadata image handling on big endian systems

Currently e2image metadata image handling and creating is completely
broken on big endian systems. It just does not care about endianness at
all. This was uncovered With addition of i_bitmaps test, which is the
first test that actually tests e2image metadata image.

Fix it by making sure that all on-disk metadata that we write and read
to/from the metadata image is properly converted.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>