Use openBackingDir() and Fstatat().
High performance impact, though part of it should be
mitigated by adding DirIV caching to the new code paths.
$ ./benchmark.bash
Testing gocryptfs at /tmp/benchmark.bash.Eou: gocryptfs v1.6-37-ge3914b3-dirty; go-fuse v20170619-66-g6df8ddc; 2018-10-14 go1.11
WRITE: 262144000 bytes (262 MB, 250 MiB) copied, 1.2289 s, 213 MB/s
READ: 262144000 bytes (262 MB, 250 MiB) copied, 1.02616 s, 255 MB/s
UNTAR: 24.490
MD5: 13.120
LS: 3.368
RM: 9.232
0c1ceed1fa