Support .read/write_iter callbacks in lieu of .aio_read/write

The aio_read and aio_write callbacks are no longer used by newer
kernels which now uses iter based readers and writers.

We can avoid implementing plain .read and .write as an iter will
be generated when needed for us automatically.

We add a new data_wait_check_iter() function accordingly.

With these methods removed from the kernel, the el8 kernel no
longer uses the extended ops wrapper struct and is much closer now
to upstream. As a result, a lot of methods are moving around from
inode_dir_operations to and from inode_file_operations etc, and
perhaps things will look a bit more structured as a result.

As a result, we need a slightly different data_wait_check() that
accounts for the iter and offset properly.

Signed-off-by: Auke Kok <auke.kok@versity.com>

kmod/src/Makefile.kernelcompat

@@ -225,3 +225,26 @@ endif
 ifneq (,$(shell grep 'readahead.*struct readahead_control' include/linux/fs.h))
 ccflags-y += -DKC_FILE_AOPS_READAHEAD
 endif
+
+#
+# v4.0-rc7-1743-g8436318205b9
+#
+# .aio_read and .aio_write no longer exist. All reads and writes now use the
+# .read_iter and .write_iter methods, or must implement .read and .write (which
+# we don't).
+#
+ifneq (,$(shell grep 'ssize_t.*aio_read' include/linux/fs.h))
+ccflags-y += -DKC_LINUX_HAVE_FOP_AIO_READ=1
+endif
+
+#
+# rhel7 has a custom inode_operations_wrapper struct that is discarded
+# entirely in favor of upstream structure since rhel8.
+#
+ifneq (,$(shell grep 'void.*follow_link.*struct dentry' include/linux/fs.h))
+ccflags-y += -DKC_LINUX_HAVE_RHEL_IOPS_WRAPPER=1
+endif
+
+ifneq (,$(shell grep 'size_t.*ki_left;' include/linux/aio.h))
+ccflags-y += -DKC_LINUX_AIO_KI_LEFT=1
+endif