251: make fstrim call a bit more random

251: make fstrim call a bit more random

To cover cases when fstrim arguments are not block/block group/file
system size aligned, we can be a bit more random. This commit changes
fstrim argument computing to use $RANDOM bash variable in order to have
different minlen, start, len argument settings and change the full fs
fstrim to be called randomly as well.

Also make kill and wait not complain about non existent process, since
it may have already finished before we attempt to kill it and wait for
it. No reason to fail the test.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>