Improve readability of copy_tree

Improve readability of copy_tree

by employing `copy mount tree from src to dst` concept.
This involves renaming the opaque variables (e.g., p, q, r, s)
to be more descriptive, aiming to make the code easier to understand.

Changes:
mnt     -> src_root (root of the tree to copy)
r       -> src_root_child (direct child of the root being cloning)
p       -> src_parent (parent of src_mnt)
s       -> src_mnt (current mount being copying)
parent  -> dst_parent (parent of dst_child)
q       -> dst_mnt (freshly cloned mount)

Signed-off-by: Jemmy <jemmywong512@gmail.com>
Link: https://lore.kernel.org/r/20240606173912.99442-1-jemmywong512@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>