Don't audit access for postinstall_mnt_dir

The dynamic linker always calls access(2) on the path. Don't generate SElinux
denials since the linker does not actually access the path in case the path
does not exist or isn't accessible for the process.

Bug: 120996057
Test: copy ping to /data/local/tmp, run it, no selinux denials
Test: bionic unit tests

Change-Id: Idf33ba7bc6c0d657b6ab0abde6bd078e4bb024e5