Fix a few typos in source code comments

- s/posbile/possible/

MFC after:	5 days

sys/cam/scsi/scsi_all.c

@@ -9150,7 +9150,7 @@ scsi_devid_match(uint8_t *lhs, size_t lhs_len, uint8_t *rhs, size_t rhs_len)
 	rhs_end = rhs + rhs_len;
 
 	/*
-	 * rhs_last and lhs_last are the last posible position of a valid
+	 * rhs_last and lhs_last are the last possible position of a valid
 	 * descriptor assuming it had a zero length identifier.  We use
 	 * these variables to insure we can safely dereference the length
 	 * field in our loop termination tests.

sys/netgraph/ng_base.c

@@ -3454,7 +3454,7 @@ ngthread(void *arg)
 
 /*
  * XXX
- * It's posible that a debugging NG_NODE_REF may need
+ * It's possible that a debugging NG_NODE_REF may need
  * to be outside the mutex zone
  */
 static void

tools/regression/netinet/ipsockopt/ipsockopt.c

@@ -163,7 +163,7 @@ test_ip_options(int sock, const char *socktypename)
 		    "returned %d bytes of data", socktypename, len);
 
 	/*
-	 * One posible failure mode is that the call succeeds but neglects to
+	 * One possible failure mode is that the call succeeds but neglects to
 	 * copy out the data.
  	 */
 	if (test_options[0] == TEST_MAGIC)