multipath: fix majmin_to_mpath_dev()

multipath: fix majmin_to_mpath_dev()

* Multipath device names only start with the mpath-prefix if the option
  use_userfriendly_names is set true in /etc/multipath.conf and if user
  has not set any aliases in the said file. Thus the for-loop should go
  through all files in /dev/mapper/, not just ones starting with 'mpath'

* Bash is perfectly capable to extend `/dev/mapper/*` notation without a
  need to pass it to an external ls

* Changed the function to use a local variable $_dev instead of the
  global $dev, which seemed to be the original intention as the local
  _dev was defined but not used