perf: Copy file mode with chmod instead of stat and write_stat

[tomciopp]

copy_file_mode/2 read the destination's file_info purely to rebuild
a full record and write it back with only :mode changed. The
destination stat served no purpose: :file.change_mode/2 is
implemented as write_file_info with a mode-only record, so applying
the source mode directly performs the same operation with one fewer
file-server round trip per file copied by File.cp/3 and File.cp_r/3.

Writing the full record back also had two unintended effects that
this removes: it attempted to set the destination's owner, which
POSIX permits to fail with EPERM for non-owner copies even when the
value is unchanged, and it rewrote the destination's timestamps with
the second-precision values read by stat, truncating the sub-second
mtime of the freshly written file.

Assisted by: Claude Fable.

[tomciopp]

Filesystem benchmarks on macOS were too noisy to certify the wall clock effect