Revert "vvfat: fix ubsan issue in create_long_filename"

Revert "vvfat: fix ubsan issue in create_long_filename"

This reverts commit 0cb3ff7c22671aa1e1e227318799ccf6762c3bea.

The original code was right in that long name in LFN directory
entry uses other parts of the entry for the name too, not just
the original "name" field.  So it is wrong to limit the offset
to be within the name field.  Some other mechanism is needed
to fix the ubsan report and whole messy usage of bytes past the
given field.

Reported-by: Volker Rümelin <vr_qemu@t-online.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>