Add comment to accept1() that it should use getsock() instead of fgetsock()

to avoid additional mutex operations, and also to avoid use of soref/sorele
which are now not preferred.

MFC after:	3 months

sys/kern/uipc_syscalls.c

@@ -283,6 +283,9 @@ done:
 /*
  * accept1()
  * MPSAFE
+ *
+ * XXXRW: Use getsock() instead of fgetsock() here to avoid additional mutex
+ * operations due to soref()/sorele().
  */
 static int
 accept1(td, uap, compat)