fix(dracut-functions): implement a cache for get_maj_min

fix(dracut-functions): implement a cache for get_maj_min

On systems with a large number of devices, usually multipath devices,
dracut can spend a lot of time stat'ing the devices to collect the
major/minor numbers, leading to huge slowness rebuilding the initramfs
when stat'ing devices is slow (seen with oracleasm file systems in
particular).
This commit implements a basic cache stored in a file under
DRACUT_TMPDIR storing the major:minor corresponding to the specified
device.

Reproducer: create N loopback devices used as a LVM extension to volume
group hosting the root file system

  # LVMVG="rhel"
  # NDEVICES=200
  # mkdir devices; for i in $(seq 1 $NDEVICES); do
    truncate -s 10m devices/$i; losetup loop$i devices/$i
  done
  # vgextend $LVMVG $(/bin/ls -1 /dev/loop[0-9]*)

With standard code (tested with RHEL8.3 dracut):

  # dracut -f --debug /tmp/initramfs.img $(uname -r) >/tmp/debug 2>&1
  # grep -c "stat -L -c" /tmp/debug
  2440

With this code:

  # dracut -f --debug /tmp/initramfs.img $(uname -r) >/tmp/debug_optim 2>&1
  # grep -c "stat -L -c" /tmp/debug_optim
  205

Signed-off-by: Renaud Métrich <rmetrich@redhat.com>