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.

Change-Id: I5a465c3a740c4fe097068ec30d2cea00aa4bb457
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>
Git-commit: 801a76050bcf8d4e500eb8d048ff6265f37a61c8
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Matt Wagantall <mattw@codeaurora.org>