sysctl: fix kernel-doc format warning

sysctl: fix kernel-doc format warning

Describe the "type" struct member using '@type' and move it together
with the rest of the doc for ctl_table_header to avoid a kernel-doc
warning:

Warning: include/linux/sysctl.h:178 Incorrect use of kernel-doc format:
 * enum type - Enumeration to differentiate between ctl target types

Fixes: 2f2665c13af4 ("sysctl: replace child with an enumeration")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Joel Granados <joel.granados@kernel.org>