vvfat: create_long_filename: fix out-of-bounds array access

vvfat: create_long_filename: fix out-of-bounds array access

create_long_filename() intentionally uses direntry_t->name[8+3] array
as a larger array.  This works, but makes static code analysis tools
unhappy.  The problem here is that a directory entry holding long file
name is significantly different from regular directory entry, and the
name is split into several parts within the entry, not just in regular
8+3 name field.

Treat the entry as array of bytes instead.  This fixes the OOB access
from the compiler/tools PoV, but does not change the resulting code
in any way.

Keep the existing code style.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>