EVM: unlock on error path in evm_read_xattrs()

We need to unlock before returning on this error path.

Fixes: fa516b66 ("EVM: Allow runtime modification of the set of verified xattrs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>