freebsd/libshare: nfs: make nfs_is_shared() thread-safe

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11886

lib/libshare/os/freebsd/nfs.c

@@ -259,14 +259,10 @@ nfs_disable_share(sa_share_impl_t impl_share)
 	    ZFS_EXPORTS_LOCK, ZFS_EXPORTS_FILE, NULL, impl_share));
 }
 
-/*
- * NOTE: This function returns a static buffer and thus is not thread-safe.
- */
 static boolean_t
 nfs_is_shared(sa_share_impl_t impl_share)
 {
-	static char line[MAXLINESIZE];
-	char *s, last;
+	char *s, last, line[MAXLINESIZE];
 	size_t len;
 	char *mntpoint = impl_share->sa_mountpoint;
 	size_t mntlen = strlen(mntpoint);