copy_file_range: require CAP_SEEK capability

When using copy_file_range(2) with an offset parameter,
the CAP_SEEK capability should be required.
This requirement is similar to the behavior observed with
pread(2)/pwrite(2).

Reported by: theraven
Reviewed by: emaste, theraven, kib, markj
Approved by: secteam
Differential Revision: https://reviews.freebsd.org/D41967