If sys/param.h MAXPHYS has been tuned to exceed MFI_MAXPHYS, the mfi(4)
real JBOD mode (SYS PD) would fail fairly reliably during I/O.

Steal the mfi_disk.c check for this condition (indirectly) when establishing
d_maxsize.

Reviewed by:	ambrisko@
MFC after:	4 weeks
Sponsored by:	Yahoo! Inc.