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[BUG]
Test case generic/475 have a very high chance (almost 100%) to hit a fs
hang, where a data page will never be unlocked and hang all later
operations.

[CAUSE]
In btrfs_do_readpage(), if we hit an error from submit_extent_page() we
will try to do the cleanup for our current io range, and exit.

This works fine for PAGE_SIZE == sectorsize cases, but not for subpage.

For subpage btrfs_do_readpage() will lock the full page first, which can
contain several different sectors and extents:

 btrfs_do_readpage()
 |- begin_page_read()
 |  |- btrfs_subpage_start_reader();
 |     Now the page will have PAGE_SIZE / sectorsize reader pending,
 |     and the page is locked.
 |
 |- end_page_read() for different branches
 |  This function will reduce subpage readers, and when readers
 |  reach 0, it will unlock the page.

But when submit_extent_page() failed, we only cleanup the current
io range, while the remaining io range will never be cleaned up, and the
page remains locked forever.

[FIX]
Update the error handling of submit_extent_page() to cleanup all the
remaining subpage range before exiting the loop.

Please note that, now submit_extent_page() can only fail due to
sanity check in alloc_new_bio().

Thus regular IO errors are impossible to trigger the error path.

CC: stable@vger.kernel.org # 5.15+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>