lib: do_strncpy_from_user: Fix return error code for get_user failures

If byte wise copy fails here, we should return EFAULT. Returning a value
other than that would cause failure of some of the user-space test cases.

CRs-Fixed: 989314
Change-Id: I38ce12d44f25dc89bdd29e8abacd8777f0a8b9a1
Signed-off-by: Imran Khan <kimran@codeaurora.org>