mnt: Optimize mntinfo_add_list to O(1) using a tail pointer

[ankushT369]

This patch optimizes the mntinfo_add_list function from O(n) to O(1) by introducing a tail pointer (tailbuffer) to track the end of the linked list. Previously, adding a new mount_info node required traversing the entire list to find the tail, which was inefficient for large lists.

Changes:

1. Added a struct list_head tail to the mount_info struct to maintain the tail pointer.
2. Introduced a global tailbuffer to store the current tail of the list.
3. Modified mntinfo_add_list to use tailbuffer for O(1) insertion at the tail.
4. Updated mntinfo_add_list_before to initialize the tail pointer and maintain consistency when adding nodes at the head.

The optimization significantly improves performance for operations involving large lists of mount information. This change also ensures that the list remains consistent and correctly updated during insertions.

[ankushT369]

@avagin can you please review this PR?

[mihalicyn]

Hi @ankushT369,

as far as I understand you work on the same fix in #2593
Why you created a second PR?

[mihalicyn]

hint: you can use git rebase -i command to interactively edit your commits and then git push -f to update your existing PR the way you want.

[mihalicyn]

if you add this struct list_head member here, then you don't need to have struct mount_info *next member anymore as you can use tail.next to access to the next struct mount_info pointer, isn't it?

[ankushT369]

@mihalicyn ok I will!!