erofs: fix PSI memstall accounting

Max Kellermann recently reported psi_group_cpu.tasks[NR_MEMSTALL] is
incorrect in the 6.11.9 kernel.

The root cause appears to be that, since the problematic commit, bio
can be NULL, causing psi_memstall_leave() to be skipped in
z_erofs_submit_queue().

Reported-by: Max Kellermann <max.kellermann@ionos.com>
Closes: https://lore.kernel.org/r/CAKPOu+8tvSowiJADW2RuKyofL_CSkm_SuyZA7ME5vMLWmL6pqw@mail.gmail.com
Fixes: 9e2f9d3 ("erofs: handle overlapped pclusters out of crafted images properly")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20241127085236.3538334-1-hsiangkao@linux.alibaba.com

Author: hsiangkao

fs/erofs/zdata.c

@@ -1792,9 +1792,9 @@ static void z_erofs_submit_queue(struct z_erofs_decompress_frontend *f,
 			erofs_fscache_submit_bio(bio);
 		else
 			submit_bio(bio);
-		if (memstall)
-			psi_memstall_leave(&pflags);
 	}
+	if (memstall)
+		psi_memstall_leave(&pflags);
 
 	/*
 	 * although background is preferred, no one is pending for submission.