Let zfs mount all tolerate in-progress mounts

The zfs-mount service can unexpectedly fail to start when zfs 
encounters a mount that is in progress. This service uses 
zfs mount -a, which has a window between the time it checks if 
the dataset was mounted and when the actual mount (via mount.zfs 
binary) occurs.

The reason for the racing mounts is that both zfs-mount.target 
and zfs-share.target are allowed to execute concurrently after 
the import.  This is more of an issue with the relatively recent 
addition of parallel mounting, and we should consider serializing
the mount and share targets.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Allan Jude <allanjude@freebsd.org>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #8881

cmd/zfs/zfs_main.c

@@ -6580,8 +6580,25 @@ share_mount_one(zfs_handle_t *zhp, int op, int flags, char *protocol,
 			return (1);
 		}
 
-		if (zfs_mount(zhp, options, flags) != 0)
+		if (zfs_mount(zhp, options, flags) != 0) {
+			/*
+			 * Check if a mount sneaked in after we checked
+			 */
+			if (!explicit &&
+			    libzfs_errno(g_zfs) == EZFS_MOUNTFAILED) {
+				usleep(10 * MILLISEC);
+				libzfs_mnttab_cache(g_zfs, B_FALSE);
+
+				if (zfs_is_mounted(zhp, NULL)) {
+					(void) fprintf(stderr, gettext(
+					    "Ignoring previous 'already "
+					    "mounted' error for '%s'\n"),
+					    zfs_get_name(zhp));
+					return (0);
+				}
+			}
 			return (1);
+		}
 		break;
 	}