tools/rmmod: fix garbled error message

tools/rmmod: fix garbled error message

a6f9cd0 ("tools/rmmod: consistently use ERR logging facility") fixed
the split between syslog and stderr of various error message substrings
by calling the ERR macro instead of writing directly to stderr, but in
doing so also completely mangled the output because the ERR macro
decorates its arguments:

    $ rmmod iwlwifi
    rmmod: ERROR: Module iwlwifi is in use by:rmmod: ERROR:  iwlmvmrmmod: ERROR:

And in syslog:

    $ rmmod -s iwlwifi
    2025-03-24T17:22:34.878318+01:00 mangolassi rmmod: ERROR: Module iwlwifi is in use by:
    2025-03-24T17:22:34.889145+01:00 mangolassi rmmod: ERROR:  iwlmvm
    2025-03-24T17:22:34.889224+01:00 mangolassi rmmod: ERROR:

This commit fixes that by building the holder names list with a strbuf
and then passes the whole thing at once to ERR.

Fixes: a6f9cd0 ("tools/rmmod: consistently use ERR logging facility")
Signed-off-by: Yannick Le Pennec <yannick.lepennec@live.fr>
Link: https://lore.kernel.org/r/VI1PR02MB48291E3BDB7E764ED67154949BA42@VI1PR02MB4829.eurprd02.prod.outlook.com
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/328