security: Fix ret values doc for security_inode_init_security()

Commit 6bcdfd2c ("security: Allow all LSMs to provide xattrs for
inode_init_security hook") unified the !initxattrs and initxattrs cases. By
doing that, security_inode_init_security() cannot return -EOPNOTSUPP
anymore, as it is always replaced with zero at the end of the function.

Also, mentioning -ENOMEM as the only possible error is not correct. For
example, evm_inode_init_security() could return -ENOKEY.

Fix these issues in the documentation of security_inode_init_security().

Fixes: 6bcdfd2c ("security: Allow all LSMs to provide xattrs for inode_init_security hook")
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>