csky: use generic strncpy/strnlen from_user

Remove the csky implemenation of strncpy/strnlen and instead use the
generic versions.  The csky version is fairly slow because it always does
byte accesses even for aligned data, and it lacks a checks for
user_addr_max().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>