libnvpair.c: replace strstr() with strchr() for a single character

Since we're looking for a single new-line character in the haystack,
it's better (and slightly more efficient) to use strchr() instead of
strstr().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: rilysh <nightquick@proton.me>
Closes #15798

lib/libnvpair/libnvpair.c

@@ -383,7 +383,7 @@ nvlist_prtctl_setfmt(nvlist_prtctl_t pctl, enum nvlist_prtctl_fmt which,
 			pctl->nvprt_btwnarrfmt_nl = 0;
 		} else {
 			pctl->nvprt_btwnarrfmt = fmt;
-			pctl->nvprt_btwnarrfmt_nl = (strstr(fmt, "\n") != NULL);
+			pctl->nvprt_btwnarrfmt_nl = (strchr(fmt, '\n') != NULL);
 		}
 		break;