sfxge(4): add efsys macro to get memory region size

EFSYS_MEM_SIZE() reports the DMA mapped size of an efsys_mem_t
allocated region (the allocation size may be different due to
memory allocator and DMA alignment restrictions).

This ensures that common code internals have explicit knowledge
of the usable size of DMA mapped memory regions.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18170