Add some VFS helpers

[brenns10]

• A corelens module "ls" for listing dentry children.
• A corelens module "fsnotify" for printing fsnotify info about every fanotify, inotify, and dnotify group.
• Some minor improvements to bt() in order to improve the fsnotify helper.

[brenns10]

Sample inotify and dnotify output from my laptop:

[PID 4071095 COMM: gvfsd-trash inotify FD 10]
FSNOTIFY GROUP: ffff8bf0b8574000
  MARK: inode ffff8bee62d7b408 /run/mount/utab.event
    CNT:2 MASK:CLOSE_WRITE|UNMOUNT IGN:0
OBJECT SUMMARY: inode: 1
0 notifications are pending.
Tasks waiting for notification:
  [EVENTPOLL: ffff8bee40280b40]
  Waiting in file->poll():
    [PID: 4071095 COMM: gvfsd-trash WAIT: select]

[SYSTEM DNOTIFY GROUP]
FSNOTIFY GROUP: ffff8bee427b5000
OBJECT SUMMARY:
0 notifications are pending.
Tasks waiting for notification:
  <no waiters>

Sample fanotify output:

[PID 3756 COMM: ds_am fanotify FD 7]
FSNOTIFY GROUP: ffff9bbd820b9d00
  MARK: inode ffff9bc1228d9640 /var/cache/uptrack/Linux/x86_64/5.15.0-206.153.7.el8uek.x86_64/#2 SMP Thu May 9 15:52:29 PDT 2024/results.server-stamp
    CNT:1 MASK:0 IGN:0
  MARK: vfsmount ffff9bbd8b244a00 FS:xfs MOUNT:/
    CNT:76 MASK:CLOSE_WRITE|OPEN_PERM|OPEN_EXEC_PERM IGN:0
   ... note: several lines removed here for brevity ...
OBJECT SUMMARY: inode: 131221, vfsmount: 32
84 notifications are pending.
Tasks waiting for notification:
  <no waiters>
0 pending permission responses
Tasks waiting for permission response from userspace:
  [PID: 1828676 COMM: sshd WAIT: direct]
  ... several lines removed ...

With higher levels of verbosity, at level 3, all of the ~80 tasks which were waiting for a permission response would have gotten printed with a stack trace. At level 4, all 131k inode marks would hav ebeen printed (by default, we only print the first 10 inodes, but we print all super blocks and vfsmounts).

Of course, for the "ls" module it's a bit less interesting.

>>> cl("ls /")
00000 /__init__.py
00001 /conftest.py
00002 /pytest.ini
00003 /tox.ini
[...]
00439 /sys
00440 /usr
00441 /etc
23 positive, 419 negative dentries
>>> cl("ls -c /")
23 positive, 419 negative dentries

The ls module can be quite slow at times due to the implementation of path_lookup() which iterates over child lists rather than using the hash table. I'll try to upstream a helper that can use the hash table as well.

[brenns10]

There's a bit of overlap in one of my waitqueue functions and the epoll code that @imran-kn has in #83, I'll see if there is anything that can be shared between these branches.

[biger410]

One comment, other looks good.

[brenns10]

Gitlab vmcore & VM tests pass!

[biger410]

Looks good.