MDEV-12353 deliberately tries to avoid memory alignment overhead in
log_phys_t, storing the stream of log records bytes straight after
a header object. But, offsetof() is not allowed on C++ data objects,
and neither are attempts to emulate it by invoking a member function
on a null pointer.

log_phys_t::len: Remove. Make it part of the byte stream that
immediately follow the object. Thanks to Eugene Kosov for this idea.

log_phys_t::start(): The start address of the following byte stream.

log_phys_t::len(): Compute len.

log_phys_t::alloc_size(): Use a simple sizeof calculation.