evm: call dump_security_xattr() in all cases to remove code duplication

Currently dump_security_xattr() is used to dump security xattr value
which is larger than 64 bytes, otherwise, pr_debug() is used. In order
to remove code duplication, refactor dump_security_xattr() and call it
in all cases.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>