Linux 4.20 compat: wrapper function for iov_iter type access

An iov_iter_type() function to access the "type" member of the struct
iov_iter was added at one point. Move the conditional logic to decide
which method to use for accessing it into a macro and simplify the
zpl_uio_init code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Coleman Kane <ckane@colemankane.org>
Closes #15100

include/os/linux/spl/sys/uio.h

@@ -179,4 +179,10 @@ zfs_uio_iov_iter_init(zfs_uio_t *uio, struct iov_iter *iter, offset_t offset,
 #define	zfs_uio_iter_iov(iter)	(iter)->iov
 #endif
 
+#if defined(HAVE_IOV_ITER_TYPE)
+#define	zfs_uio_iov_iter_type(iter)	iov_iter_type((iter))
+#else
+#define	zfs_uio_iov_iter_type(iter)	(iter)->type
+#endif
+
 #endif /* SPL_UIO_H */

module/os/linux/zfs/zpl_file.c

@@ -300,13 +300,10 @@ zpl_uio_init(zfs_uio_t *uio, struct kiocb *kiocb, struct iov_iter *to,
 {
 #if defined(HAVE_VFS_IOV_ITER)
 	zfs_uio_iov_iter_init(uio, to, pos, count, skip);
-#elif defined(HAVE_IOV_ITER_TYPE)
-	zfs_uio_iovec_init(uio, zfs_uio_iter_iov(to), to->nr_segs, pos,
-	    iov_iter_type(to) & ITER_KVEC ? UIO_SYSSPACE : UIO_USERSPACE,
-	    count, skip);
 #else
 	zfs_uio_iovec_init(uio, zfs_uio_iter_iov(to), to->nr_segs, pos,
-	    to->type & ITER_KVEC ? UIO_SYSSPACE : UIO_USERSPACE,
+	    zfs_uio_iov_iter_type(to) & ITER_KVEC ?
+	    UIO_SYSSPACE : UIO_USERSPACE,
 	    count, skip);
 #endif
 }