ZTS: Debug zfs_share_concurrent_shares failure

Update zfs_share_concurrent_shares test case to wait a few seconds
and recheck that the filesystem isn't shared.  The intent here is
determine the nature of the error and if it may be a race.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

tests/zfs-tests/tests/functional/cli_root/zfs_share/zfs_share_concurrent_shares.ksh

@@ -98,8 +98,17 @@ function test_share # filesystem
 
 	zfs set sharenfs=on $filesystem || \
 	    sub_fail "zfs set sharenfs=on $filesystem failed."
-	is_shared $mntp || \
-	    sub_fail "File system $filesystem is not shared (set sharenfs)."
+
+	for retry in $(seq 1 10); do
+		is_shared $mntp && return
+		log_note "Wait $retry / 10 for is_shared $mntp"
+
+		if [[ $retry -eq 10 ]]; then
+			sub_fail "File system $filesystem is not shared (set sharenfs)."
+		fi
+
+		sleep 1
+	done
 
 	#
 	# Verify 'zfs share' works as well.