base: genlock: add magic to protect attach from non-genlock file

It is possible for user space to attempt to attach a valid fd that
does not correspond to a genlock file. This change adds a magic to
the kernel private data that is used for validation.

CRs-fixed: 359649
Signed-off-by: Jeff Boody <jboody@codeaurora.org>
(cherry picked from commit 507a9d3354b24e7cc7f22e9695339de6df317fc1)

Change-Id: Ice93bfc904f61431688f03027a4090ceb7b9289a
Signed-off-by: Ramakrishna Prasad N <crpn@codeaurora.org>
(cherry picked from commit 6856721e05a2e97130a654884a16cf8d79753d58)

Signed-off-by: Sudhir Sharma <sudsha@codeaurora.org>

Conflicts:
	drivers/base/genlock.c