sysctl: Replace unidirectional INT converter macros with functions

sysctl: Replace unidirectional INT converter macros with functions

Replace SYSCTL_USER_TO_KERN_INT_CONV and SYSCTL_KERN_TO_USER_INT_CONV
macros with function implementing the same logic.This makes debugging
easier and aligns with the functions preference described in
coding-style.rst. Update all jiffies converters to use explicit function
implementations instead of macro-generated versions.

Signed-off-by: Joel Granados <joel.granados@kernel.org>