file: Add foffset_lock_pair()

This will be used in kern_copy_file_range(), which needs to lock two
offsets.

Reviewed by: kib, rmacklem
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D49440

(cherry picked from commit 12ecb0fe0afda8c051605045e446371ddd34741f)