Revert "Let zfs mount all tolerate in-progress mounts"

This reverts commit a9cd8bf which introduced a segfault when running
`zfs mount -a` multiple times when there are mountpoints which are
not empty.  This segfault is now seen frequently by the CI after
the mount code was updated to directly call mount(2).

The original reason this logic was added is described in openzfs#8881.
Since then the systemd `zfs-share.target` has been updated to run
"After" the `zfs-mount.server` which should avoid this issue.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue openzfs#9560

cmd/zfs/zfs_main.c

@@ -6734,25 +6734,8 @@ share_mount_one(zfs_handle_t *zhp, int op, int flags, char *protocol,
 			return (1);
 		}
 
-		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);
-				}
-			}
+		if (zfs_mount(zhp, options, flags) != 0)
 			return (1);
-		}
 		break;
 	}