hexagon: use generic strncpy/strnlen from_user

Remove the hexagon implementation of strncpy/strnlen and instead use
the generic version.  The hexagon version reads the data twice for
strncpy() by doing an extra strnlen(), and it apparently lacks a check
for user_addr_max().

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