libshare: nfs: always try to mkdir()

This also works out to one syscall if the directory exists,
but is one syscall shorter if it doesn't.

Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes openzfs#12067

lib/libshare/nfs.c

@@ -89,11 +89,9 @@ struct tmpfile {
 static boolean_t
 nfs_init_tmpfile(const char *prefix, const char *mdir, struct tmpfile *tmpf)
 {
-	struct stat sb;
-
 	if (mdir != NULL &&
-	    stat(mdir, &sb) < 0 &&
-	    mkdir(mdir, 0755) < 0) {
+	    mkdir(mdir, 0755) < 0 &&
+	    errno != EEXIST) {
 		fprintf(stderr, "failed to create %s: %s\n",
 		    mdir, strerror(errno));
 		return (B_FALSE);