Btrfs: clean up scrub is_dev_replace parameter

[ Upstream commit 32934280967d00dc2b5c4d3b63b21a9c8638326e ]

struct scrub_ctx has an ->is_dev_replace member, so there's no point in
passing around is_dev_replace where sctx is available.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>