evm: terminate and bound the evm_xattrs read buffer

In the Linux kernel, the following vulnerability has been resolved: evm: terminate and bound the evm_xattrs read buffer evm_read_xattrs() allocates size + 1 bytes, fills them from the list of enabled xattrs, and then passes strlen(temp) to simple_read_from_buffer(). When no configured xattrs are enabled, the fill loop stores nothing and temp[0] remains uninitialized, so strlen() reads beyond initialized memory. Explicitly terminate the buffer after allocation, use snprintf() for each formatted line, and pass the accumulated length, without risk of truncation, to simple_read_from_buffer().