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.

modules.d/99fs-lib/fs-lib.sh

@@ -44,22 +44,22 @@ fsck_able() {
             ;;
         ext?)
             type e2fsck >/dev/null 2>&1 &&
-            _drv="_drv=e2fsck fsck_drv_com" &&
+            _drv="fsck_drv_com e2fsck" &&
             return 0
             ;;
         f2fs)
 	    type fsck.f2fs >/dev/null 2>&1 &&
-	    _drv="_drv=fsck.f2fs fsck_drv_com" &&
+	    _drv="fsck_drv_com fsck.f2fs" &&
 	    return 0
 	    ;;
         jfs)
             type jfs_fsck >/dev/null 2>&1 &&
-            _drv="_drv=jfs_fsck fsck_drv_com" &&
+            _drv="fsck_drv_com jfs_fsck" &&
             return 0
             ;;
         reiserfs)
             type reiserfsck >/dev/null 2>&1 &&
-            _drv="_drv=reiserfsck fsck_drv_com" &&
+            _drv="fsck_drv_com reiserfsck" &&
             return 0
             ;;
         btrfs)
@@ -70,12 +70,12 @@ fsck_able() {
             ;;
         nfs*)
             # nfs can be a nop, returning success
-            _drv="_drv=none :" &&
+            _drv=":" &&
             return 0
             ;;
         *)
             type fsck >/dev/null 2>&1 &&
-            _drv="_drv=fsck fsck_drv_std" &&
+            _drv="fsck_drv_std fsck" &&
             return 0
             ;;
     esac
@@ -97,6 +97,7 @@ fsck_drv_btrfs() {
 
 # common code for checkers that follow usual subset of options and return codes
 fsck_drv_com() {
+    local _drv="$1"
     local _ret
     local _out