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: so
Security: FreeBSD-SA-23:13.capsicum
Security: CVE-2023-5369
Differential Revision: https://reviews.freebsd.org/D41967

(cherry picked from commit 15a51d3abaef27ddea66320cac7caa549738a1a6)
(cherry picked from commit 3f0ce63828dc3d4030b32ad0effb4e588af49c03)