btrfs: scrub: always update btrfs_scrub_progress::last_physical

[BUG]
When a scrub failed immediately without any byte scrubbed, the returned
btrfs_scrub_progress::last_physical will always be 0, even if there is a
non-zero @start passed into btrfs_scrub_dev() for resume cases.

This will reset the progress and make later scrub resume start from the
beginning.

[CAUSE]
The function btrfs_scrub_dev() accepts a @progress parameter to copy its
updated progress to the caller, there are cases where we either don't
touch progress::last_physical at all or copy 0 into last_physical:

- last_physical not updated at all
If some error happened before scrubbing any super block or chunk, we
will not copy the progress, leaving the @last_physical untouched.

E.g. failed to allocate @sctx, scrubbing a missing device or even
there is already a running scrub and so on.

All those cases won't touch @progress at all, resulting the
last_physical untouched and will be left as 0 for most cases.

- Error out before scrubbing any bytes
In those case we allocated @sctx, and sctx->stat.last_physical is all
zero (initialized by kvzalloc()).
Unfortunately some critical errors happened during
scrub_enumerate_chunks() or scrub_supers() before any stripe is really
scrubbed.

In that case although we will copy sctx->stat back to @progress, since
no byte is really scrubbed, last_physical will be overwritten to 0.

[FIX]
Make sure the parameter @progress always has its @last_physical member
updated to @start parameter inside btrfs_scrub_dev().

At the very beginning of the function, set @progress->last_physical to
@start, so that even if we error out without doing progress copying,
last_physical is still at @start.

Then after we got @sctx allocated, set sctx->stat.last_physical to
@start, this will make sure even if we didn't get any byte scrubbed, at
the progress copying stage the @last_physical is not left as zero.

This should resolve the resume progress reset problem.

Signed-off-by: Qu Wenruo <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/scrub.c

···
 	unsigned int nofs_flag;
 	bool need_commit = false;
 
+	/* Set the basic fallback @last_physical before we got a sctx. */
+	if (progress)
+		progress->last_physical = start;
+
 	if (btrfs_fs_closing(fs_info))
 		return -EAGAIN;
 
···
 	sctx = scrub_setup_ctx(fs_info, is_dev_replace);
 	if (IS_ERR(sctx))
 		return PTR_ERR(sctx);
+	sctx->stat.last_physical = start;
 
 	ret = scrub_workers_get(fs_info);
 	if (ret)