Revert "exfat: limit backup volume name to 11 characters"

This reverts commit c79381d32a61a570bff305744b1230100c735e22.
exfat: fix problems with hrev46820

* Set the size of the volume name member var to B_FILE_NAME_LENGTH which is the
  max width of a volume name according to the BeBook and the Support Kit code. We'll
  deal with trying to stuff the volume name back into the exfat volume label later.
* Rename volume_name() to get_volume_name()
* Pass the name string length into get_volume_name() and use it to avoid buffer overrun.
* Fill name with a blank string if volume has no label.
* Don't memset the name with zeros before doing the conversion from Unicode to UTF-8,
  the conversion function will make sure the result is NUL-terminated if it returns B_OK.
* Also check the return value of the Unicode conversion function and return an error if it fails.
* Add get_default_volume_name() method to Utility.cpp which is used to fill out the default
  volume name in the case volume name is blank. e.g. 32GiB ExFAT Volume. This now
  applies to both the volume name and mount point.
* Use non-metric prefixes for default volume name, e.g. MiB, GiB, TiB
* For an unset volume name fill the volume name with an empty string.
* Remove the leading underscore from _name and _partition parameters
* Replace size constants with sizeof() calls
* Remove Axel from the copyright statement in Utility.cpp, he had nothing to do with it,
  add Jérôme (aka korli) because he wrote the code for get_default_volume_name()
  (was in Volumes.cpp)
* Remove some trailing spaces from encodings.cpp

Thanks Axel.
exfat: small correction in comment, name not _name
exfat: recheck device size is big enough to hold fs

I didn't mean to omit this check, I just need to save deviceSize below for
get_default_volume_name() so that it will match the mount point name.

Checked to make sure the calculation works on a 2GiB, 32GiB, and 128GiB
(virtual) exfat volume.