Fix gcc compiler warning, parse_option()

When compiling ZFS in user space gcc-4.6.0 correctly identifies
the variable 'value' as being set but never used.  This generates a
warning and a build failure when using --enable-debug.  Once again
this is correct but I'm reluctant to remove 'value' because we are
breaking the string in to name/value pairs.  While it is not used
now there's a good chance it will be soon and I'd rather not have
to reinvent this.  To suppress the warning with just as a VERIFY().
This was observed under Fedora 15.

  cmd/mount_zfs/mount_zfs.c: In function ‘parse_option’:
  cmd/mount_zfs/mount_zfs.c:112:21: error: variable ‘value’ set but not
  used [-Werror=unused-but-set-variable]

cmd/mount_zfs/mount_zfs.c

@@ -120,6 +120,7 @@ parse_option(char *mntopt, unsigned long *mntflags,
 		if (*ptr == '=') {
 			*ptr = '\0';
 			value = ptr+1;
+			VERIFY3P(value, !=, NULL);
 			break;
 		}
 	}