seq_file: always clear m->count when we free m->buf

Once we'd freed m->buf, m->count should become zero - we have no valid
contents reachable via m->buf.

cherry-picked from https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/fs/seq_file.c?id=801a76050bcf8d4e500eb8d048ff6265f37a61c8

Change-Id: I4c1d3e69db4ecf5362e2a5d05bfd7db754dc6dc6
Reported-by: Charley (Hao Chuan) Chu <charley.chu@broadcom.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Devin Kim <dojip.kim@lge.com>