lib/blkid: fix unaligned access to hfs_mdb

lib/blkid: fix unaligned access to hfs_mdb

With -Wall, gcc warns:

      ./probe.c:1209:42: error: taking address of packed member of
               'struct hfs_mdb' may result in an unaligned pointer value

This seems to be a real unaligned memory access bug, as the offset of
the 64-bit value from the start of the buffer is 116, which is not a
multiple of 8.  Fix it by using memcpy().

Do the same for hfsplus to fix the same warning, though in that case the
offset is a multiple of 8 so it was defined behavior.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>