EVM: fix memory leak of temporary buffer 'temp'

The allocation of 'temp' is not kfree'd and hence there is a memory
leak on each call of evm_read_xattrs.  Fix this by kfree'ing it
after copying data from it back to the user space buffer 'buf'.

Detected by CoverityScan, CID#1469386 ("Resource Leak")

Fixes: fa516b66 ("EVM: Allow runtime modification of the set of verified xattrs")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>