Merge branch 'netdev-seq_file-next-functions-should-increase-position-index'

Vasily Averin says:

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netdev: seq_file .next functions should increase position index

In Aug 2018 NeilBrown noticed
commit 1f4aace6 ("fs/seq_file.c: simplify seq_file iteration code and interface")
"Some ->next functions do not increment *pos when they return NULL...
Note that such ->next functions are buggy and should be fixed.
A simple demonstration is

dd if=/proc/swaps bs=1000 skip=1

Choose any block size larger than the size of /proc/swaps.  This will
always show the whole last line of /proc/swaps"

Described problem is still actual. If you make lseek into middle of last output line
following read will output end of last line and whole last line once again.

$ dd if=/proc/swaps bs=1  # usual output
Filename				Type		Size	Used	Priority
/dev/dm-0                               partition	4194812	97536	-2
104+0 records in
104+0 records out
104 bytes copied

$ dd if=/proc/swaps bs=40 skip=1    # last line was generated twice
dd: /proc/swaps: cannot skip to specified offset
v/dm-0                               partition	4194812	97536	-2
/dev/dm-0                               partition	4194812	97536	-2
3+1 records in
3+1 records out
131 bytes copied

There are lot of other affected files, I've found 30+ including
/proc/net/ip_tables_matches and /proc/sysvipc/*

This patch-set fixes files related to netdev@

https://bugzilla.kernel.org/show_bug.cgi?id=206283


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Signed-off-by: David S. Miller <davem@davemloft.net>