tools/opensnoop: support mount parent full-path

[Rtoax]

Like d_path()/prepend_path() [1] did, get mount parent full-path.

For example, /dev/sda mount on /home/sda:

$ lsblk
NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
sda           8:0    0   1.8T  0 disk /home/sda

and open file under /home/sda/:

$ cd /home/sda/git-repos/tst-linux
$ touch a.out

Old tracing, only get the mount root:

$ sudo ./opensnoop.py -F
PID    COMM               FD ERR PATH
114293 touch               3   0 /git-repos/test-linux/a.out

This patch get real full path:

$ sudo ./opensnoop.py -F
PID    COMM               FD ERR PATH
114110 touch               3   0 /home/sda/git-repos/test-linux/a.out
                                 ^^^^^^^^^

Link: https://github.com/torvalds/linux/blob/master/fs/d_path.c [1]

[Rtoax]

Perhaps we need to first address the issue of multiple processing of events caused by the full-path patch, which was introduced in c110a4d. Each loop iteration will send out an event, which is too much and unnecessary. cc @yonghong-song

[yonghong-song]

Sounds good. Let us resolve #5323 first and then deal with this patch.

[Rtoax]

Perhaps we need to first address the issue of multiple processing of events caused by the full-path patch, which was introduced in c110a4d. Each loop iteration will send out an event, which is too much and unnecessary. cc @yonghong-song

Close this PR and fix this first.