fs-lib: drop a bashism

fs-lib: drop a bashism

Bash 5 apparently longer propagates variable assignments to local variables
in front of function calls when in POSIX mode:

  [lkundrak@demiurge ~]$ cat feh.sh
  print_VAR () {
          echo "$VAR";
  }

  testfunc () {
          local VAR="OLD"
          VAR=NEW print_VAR
  }

  testfunc
  [lkundrak@demiurge ~]$ bash4 --posix feh.sh
  NEW
  [lkundrak@demiurge ~]$ bash5 --posix feh.sh
  OLD
  [lkundrak@demiurge ~]$ bash5 feh.sh
  NEW
  [lkundrak@demiurge ~]$

It works the way it did in Bash 4 in non-POSIX mode, for external programs,
or for non-local variables. Don't ask me why -- it's probably some
compatibility thing for some sad old people.

However, this precisely happens when fsck_single() is calling into the
fsck_drv_com(), assigned to _drv by fsck_able(). That ruins the
TEST-70-BONDBRIDGETEAMVLAN test's server and probably more.

Let's pass the fsck driver binary via the function argument instead. It's
less messy anyway.