apparmor: Fix some kernel-doc comments

Add the description of @ns_name, change function name aa_u16_chunck to
unpack_u16_chunk and verify_head to verify_header in kernel-doc comment
to remove warnings found by running scripts/kernel-doc, which is caused
by using 'make W=1'.

security/apparmor/policy_unpack.c:224: warning: expecting prototype for
aa_u16_chunck(). Prototype was for unpack_u16_chunk() instead
security/apparmor/policy_unpack.c:678: warning: Function parameter or
member 'ns_name' not described in 'unpack_profile'
security/apparmor/policy_unpack.c:950: warning: expecting prototype for
verify_head(). Prototype was for verify_header() instead

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>