vvfat: Fix usage of `info.file.offset`

vvfat: Fix usage of `info.file.offset`

The field is marked as "the offset in the file (in clusters)", but it
was being used like this
`cluster_size*(nums)+mapping->info.file.offset`, which is incorrect.

Signed-off-by: Amjad Alsharafi <amjadsharafi10@gmail.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <72f19a7903886dda1aa78bcae0e17702ee939262.1721470238.git.amjadsharafi10@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 21b25a0e466a5bba0a45600bb8100ab564202ed1)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>