mavlink_stream: mutex lock for register/unregister poll #1478

Job	Run time
variables	1s
fc_matrix (fmu-v6xrt) / build	6m 47s
fc_matrix (pixhawk) / build	5m 38s
fc_matrix (saluki-nxp93_flat) / build	5m 34s
fc_matrix (saluki-pi_amp) / build	8m 38s
fc_matrix (saluki-pi_custom_keys, Tools/saluki-sec-scripts/custom_keys/saluki-pi/px4_bin_ed25519_... / build	9m 16s
fc_matrix (saluki-pi_default) / build	9m 14s
fc_matrix (saluki-pi_flat) / build	6m 31s
fc_matrix (saluki-v2_amp) / build	8m 58s
fc_matrix (saluki-v2_custom_keys, Tools/saluki-sec-scripts/custom_keys/saluki-v2/px4_bin_ed25519_... / build	8m 51s
fc_matrix (saluki-v2_default) / build	9m 3s
fc_matrix (saluki-v2_flat) / build	6m 30s
fc_matrix (saluki-v3_amp) / build	9m 10s
fc_matrix (saluki-v3_custom_keys, Tools/saluki-sec-scripts/custom_keys/saluki-v3/px4_bin_ed25519_... / build	8m 41s
fc_matrix (saluki-v3_flat) / build	6m 33s
fc_matrix (saluki-v3_default) / build	8m 33s
build px4fwupdater	2m 20s
upload builds to UAE artifactory	3m 24s
upload px4fwupdater to docker registry	1m 25s
upload px4fwupdater to UAE docker registry	0s
	2h 5m 7s

Provide feedback