lib: use wrapper for curl_mime_data fseek callback

lib: use wrapper for curl_mime_data fseek callback

fseek uses long offset which does not match with curl_off_t. This leads
to undefined behavior when calling the callback and caused failure on
arm 32 bit.

Use a wrapper to solve this and use fseeko which uses off_t instead of
long.

Thanks to the nice people at Libera IRC #musl for helping finding this
out.

Fixes #11882
Fixes #11900
Closes #11918