2976 remove useless offsetof() macros

*** 50,59 ****
--- 50,60 ----
  #include <sys/ddi.h>
  #include <sys/sunddi.h>
  #include <sys/stream.h>         /* required for MBLK* */
  #include <sys/strsun.h>         /* required for mionack() */
  #include <sys/byteorder.h>
+ #include <sys/sysmacros.h>
  #include <sys/pci.h>
  #include <inet/common.h>
  #include <inet/led.h>
  #include <inet/mi.h>
  #include <inet/nd.h>
*** 105,117 ****
  #endif
  
  #define GET_TXBUF(dp, sn)       \
          &(dp)->tx_buf[SLOT((dp)->tx_slots_base + (sn), (dp)->gc.gc_tx_buf_size)]
  
- #ifndef offsetof
- #define offsetof(t, m)  ((long)&(((t *) 0)->m))
- #endif
  #define TXFLAG_VTAG(flag)       \
          (((flag) & GEM_TXFLAG_VTAG) >> GEM_TXFLAG_VTAG_SHIFT)
  
  #define MAXPKTBUF(dp)   \
          ((dp)->mtu + sizeof (struct ether_header) + VTAG_SIZE + ETHERFCSL)
--- 106,115 ----