reiserfs: Switch to security_inode_init_security()

In preparation for removing security_old_inode_init_security(), switch to
security_inode_init_security(). Commit 572302af ("reiserfs: Add missing
calls to reiserfs_security_free()") fixed possible memory leaks and another
issue related to adding an xattr at inode creation time.

Define the initxattrs callback reiserfs_initxattrs(), to populate the
name/value/len triple in the reiserfs_security_handle() with the first
xattr provided by LSMs. Make a copy of the xattr value, as
security_inode_init_security() frees it.

After the call to security_inode_init_security(), remove the check for
returning -EOPNOTSUPP, as security_inode_init_security() changes it to
zero.

Multiple xattrs are currently not supported, as the
reiserfs_security_handle structure is exported to user space. As a
consequence, even if EVM is invoked, it will not provide an xattr (if it
is not the first to set it, its xattr will be discarded; if it is the
first, it does not have xattrs to calculate the HMAC on).

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>