lsm: convert security_setselfattr() to use memdup_user()

As suggested by the kernel test robot, memdup_user() is a better
option than the combo of kmalloc()/copy_from_user().

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310270805.2ArE52i5-lkp@intel.com/


Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>