pft: add allow_merge_bio() hook when PFT is LSM

When PFT is the chosen-LSM (Linux Security Module) ,
and it registers to the Security-Framework,
it should provide the allow_merge_bio() callback hook.

Change-Id: I2c98536828476fc79ecaf59a2e21c19b530bd3d9
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>