Avoid small buffer copying on write

It is wrong for arc_write_ready() to use zfs_abd_scatter_enabled to
decide whether to reallocate/copy the buffer, because the answer is
OS-specific and depends on the buffer size.  Instead of that use
abd_size_alloc_linear(), moved into public header.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes #12425

module/os/linux/zfs/abd_os.c

@@ -638,7 +638,7 @@ abd_alloc_zero_scatter(void)
 boolean_t
 abd_size_alloc_linear(size_t size)
 {
-	return (size < zfs_abd_scatter_min_size ? B_TRUE : B_FALSE);
+	return (!zfs_abd_scatter_enabled || size < zfs_abd_scatter_min_size);
 }
 
 void