UPSTREAM: dma-buf: add show_fdinfo handler

The show_fdinfo handler exports the same information available through
debugfs on a per-buffer basis.

Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Chenbo Feng <fengc@google.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613223408.139221-4-fengc@google.com
(cherry picked from commit bcc071110aeacd28a87525a2442dd96eab145a3c)

Bug: 135613055
Test: run dmabufinfo_test.cpp and check the info is showed up in
      /proc/pid/fdinfo

Change-Id: I7e82209af812e2a3160ec470113c2ced991c9fc3