Starting with commit 12aceb89b0bc ("eventfd: convert to f_op->read_iter()")
in 5.8-rc1, the "read" function pointer in eventfd's file_operations is
NULL, "read_iter" is available instead.
Use kernel_read() and kernel_write() instead of directly calling the
function pointers to handle this correctly. We can do that safely for
kernel 4.14 and newer (older versions have different arguments of
kernel_read() and kernel_write()).