Fix a common typo in source code comments

- s/existant/existent/

(cherry picked from commit 631504fb34)

sys/dev/sfxge/common/efx_vpd.c

@@ -901,7 +901,7 @@ efx_vpd_hunk_set(
 
 				/*
 				 * The keyword doesn't already exist. If the
-				 * user deleting a non-existant keyword then
+				 * user deleting a non-existent keyword then
 				 * this is a no-op.
 				 */
 				if (evvp->evv_length == 0)

sys/mips/cavium/octe/ethernet-util.h

@@ -59,7 +59,7 @@ static inline int INTERFACE(int ipd_port)
 		return 2;
 	else if (ipd_port < 40)   /* Interface 3 for loopback */
 		return 3;
-	else if (ipd_port == 40)  /* Non existant interface for POW0 */
+	else if (ipd_port == 40)  /* Non existent interface for POW0 */
 		return 4;
 	else
 		panic("Illegal ipd_port %d passed to INTERFACE\n", ipd_port);

sys/netinet/sctp_output.c

@@ -12689,8 +12689,8 @@ sctp_lower_sosend(struct socket *so,
 			if ((sinfo_flags & SCTP_ABORT) ||
 			    ((sinfo_flags & SCTP_EOF) && (sndlen == 0))) {
 				/*-
-				 * User asks to abort a non-existant assoc,
-				 * or EOF a non-existant assoc with no data
+				 * User asks to abort a non-existent assoc,
+				 * or EOF a non-existent assoc with no data
 				 */
 				SCTP_LTRACE_ERR_RET(inp, stcb, net, SCTP_FROM_SCTP_OUTPUT, ENOENT);
 				error = ENOENT;

sys/rpc/svc.h

@@ -669,7 +669,7 @@ __END_DECLS
  * Somebody has to wait for incoming requests and then call the correct
  * service routine.  The routine svc_run does infinite waiting; i.e.,
  * svc_run never returns.
- * Since another (co-existant) package may wish to selectively wait for
+ * Since another (co-existent) package may wish to selectively wait for
  * incoming calls or other events outside of the rpc architecture, the
  * routine svc_getreq is provided.  It must be passed readfds, the
  * "in-place" results of a select system call (see select, section 2).