bin/setfacl: Little refactoring, no functional change.

The acl_from_stat function accepts a stat_t * argument, but only uses its
st_mode field. There is no reason to pass the whole struct, so make it accept
a mode_t and rename the function to acl_from_mode.
Linux has non-standard acl_from_mode function in its libacl, so naming the
function this way may help discovering it during porting efforts.

Reviewed by: tsoome, markj
Approved by: markj
Differential Revision: https://reviews.freebsd.org/D27292