evm: Fix build warnings

Fix build warnings (function parameters description) for
evm_read_protected_xattrs(), evm_set_key() and evm_verifyxattr().

Fixes: 76266763 ("evm: provide a function to set the EVM key from the kernel") # v4.5+
Fixes: 8314b673 ("ima: Define new template fields xattrnames, xattrlengths and xattrvalues") # v5.14+
Fixes: 2960e6cb ("evm: additional parameter to pass integrity cache entry 'iint'") # v3.2+
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>