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);

module/zfs/arc.c

@@ -6917,7 +6917,8 @@ arc_write_ready(zio_t *zio)
 		arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT | ARC_HDR_ALLOC_RDATA |
 		    ARC_HDR_USE_RESERVE);
 		abd_copy(hdr->b_crypt_hdr.b_rabd, zio->io_abd, psize);
-	} else if (zfs_abd_scatter_enabled || !arc_can_share(hdr, buf)) {
+	} else if (!abd_size_alloc_linear(arc_buf_size(buf)) ||
+	    !arc_can_share(hdr, buf)) {
 		/*
 		 * Ideally, we would always copy the io_abd into b_pabd, but the
 		 * user may have disabled compressed ARC, thus we must check the