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/zfs/abd.c

@@ -181,7 +181,7 @@ abd_free_struct(abd_t *abd)
 abd_t *
 abd_alloc(size_t size, boolean_t is_metadata)
 {
-	if (!zfs_abd_scatter_enabled || abd_size_alloc_linear(size))
+	if (abd_size_alloc_linear(size))
 		return (abd_alloc_linear(size, is_metadata));
 
 	VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);