MariaDB Galera does not expect 'mbstream' as streamfmt in the [sst] section and defaults to non-default 'tar'

Using:

[sst]
streamfmt=mbstream

causes SST to fail because the format defaults for some reason to 'tar'

A quick investigation led to find that while wsrep_sst_mariabackup uses 'mbstream' as binary for streaming, it only handles the parameter value 'xbstream'.

    if [[ $sfmt == 'xbstream' ]];then

        wsrep_log_info "Streaming with xbstream"

        if [[ "$WSREP_SST_OPT_ROLE"  == "joiner" ]];then

            strmcmd="${XBSTREAM_BIN} -x"

....

XBSTREAM_BIN=mbstream

Maybe it would be useful to have wsrep_sst_mariabackup accept both xbstream and mbstream and map them both to 'mbstream' so to assure a smooth migration for who migrates from xtrabackup sst method to mariabackup.

Or, imho, if it should only accept one value, that would be 'mbstream' in MariaDB Galera using mariabackup as sst method.