s390/uaccess: fix strncpy_from_user string length check

The "standard" and page table walk variants of strncpy_from_user() first
check the length of the to be copied string in userspace.
The string is then copied to kernel space and the length returned to the
caller.
However userspace can modify the string at any time while the kernel
checks for the length of the string or copies the string. In result the
returned length of the string is not necessarily correct.
Fix this by copying in a loop which mimics the mvcos variant of
strncpy_from_user(), which handles this correctly.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>