cifs: silence compiler warnings showing up with gcc-4.7.0

    gcc-4.7.0 has started throwing these warnings when building cifs.ko.

      CC [M]  fs/cifs/cifssmb.o
    fs/cifs/cifssmb.c: In function CIFSSMBSetCIFSACL:
    fs/cifs/cifssmb.c:3905:9: warning: array subscript is above array bounds [-Warray-bounds]
    fs/cifs/cifssmb.c: In function CIFSSMBSetFileInfo:
    fs/cifs/cifssmb.c:5711:8: warning: array subscript is above array bounds [-Warray-bounds]
    fs/cifs/cifssmb.c: In function CIFSSMBUnixSetFileInfo:
    fs/cifs/cifssmb.c:6001:25: warning: array subscript is above array bounds [-Warray-bounds]

    This patch cleans up the code a bit by using the offsetof macro instead
    of the funky "&pSMB->hdr.Protocol" construct.

    Signed-off-by: Jeff Layton <jlayton <at> redhat.com>
    Signed-off-by: Steve French <sfrench <at> us.ibm.com>