cp: make buffer local to copy_fallback()

[ehem_freebsd_m5p.com]

Summary

As the copy buffer isn't used anywhere else, it should be local instead
of being passed in each time. Previously it had been static to
copy_file().

[ehem_freebsd_m5p.com]

Erk, noticed an issue. I'm unsure of the magnitude of this.

bin/cp/utils.c

87–91

Bit of funkiness about this comment now. copy_fallback() may never get called if copy_file_range() functions for all files. copy_fallback() though could fail in the middle of copying files if copy_file_range() functions for some, but not for others.

As such, this comment isn't guaranteed to be true anymore. It seems unlikely for copy_fallback() to fail in the middle, but it is now possible. I'm tempted to leave this alone as it will still be true most of the time.