sysctl: Discriminate between kernel and user converter params

sysctl: Discriminate between kernel and user converter params

Rename converter parameter to indicate data flow direction: "lvalp" to
"u_ptr" indicating a user space parsed value pointer. "valp" to "k_ptr"
indicating a kernel storage value pointer. This facilitates the
identification of discrepancies between direction (copy to kernel or
copy to user space) and the modified variable. This is a preparation
commit for when the converter functions are exposed to the rest of the
kernel.

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