vfio/qat: fix f_pos race in qat_vf_resume_write()

In the Linux kernel, the following vulnerability has been resolved: vfio/qat: fix f_pos race in qat_vf_resume_write() qat_vf_resume_write() checks filp->f_pos before taking migf->lock, but copies into the migration-state buffer after taking the lock and re-reading the shared file position. Two concurrent writers could therefore pass the bounds check with the old offset, then have the second writer copy after the first advanced f_pos, writing past the end of the migration-state buffer. Take migf->lock before doing the boundary checks.