kernel: Use our offsetof() from <sys/types.h> instead of a local one.

kernel: Use our offsetof() from <sys/types.h> instead of a local one.

On GCC 4.1 and later, our offsetof() winds up being __builtin_offsetof(),
which is constant to GCC, so as a nice benefit this commit also fixes two
GCC 4.7 warnings:

.../dirhash.h:97:95: warning: variably modified 'dh_firstfree' at file scope
.../vdevice.h:145:2: warning: variably modified '_ArrayTables' at file scope

There are more offsetof() candidates I need to get to but since those two
fix warnings, I commit them now.