libext2fs: fix unaligned, multiblock writes in the unix_io handler

libext2fs: fix unaligned, multiblock writes in the unix_io handler

The read-modify-write code for the unaligned fallback code wasn't
working for multi-block writes.  This was unmasked by FreeBSD 11-rc2,
since its malloc() is returning unaligned memory regions for large
memory regions.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>