sh: Implement __get_user_u64() required for 64-bit get_user()

Trying to build the kernel with CONFIG_INFINIBAND_USER_ACCESS enabled fails

     ERROR: "__get_user_unknown" [drivers/infiniband/core/ib_uverbs.ko] undefined!

with on SH since the kernel misses a 64-bit implementation of get_user().

Implement the missing 64-bit get_user() as __get_user_u64(), matching the
already existing __put_user_u64() which implements the 64-bit put_user().

Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Acked-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Rich Felker <dalias@libc.org>