find(1): Don't treat statfs() error as fatal in f_fstype, which can happen
when a directory is removed in the middle of find.

Instead of a full err(), allow find to continue, plus print a warning with
exitstatus set when appropriate.

Reported by: 100.chksetuid via gordon
Reviewed by: jilles
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D13024