[media] videobuf2-core: Add null check for current->mm in buf_prepare

If current->mm is NULL, then buf_prepare ends up using
garbage semaphor value from current->mm->mmap_sem during
down_read. This results in a device crash. This change
fixes the issue.

Change-Id: I9096c7d674d2493def1ce57ece551e7cd8f9e298
Signed-off-by: Arun Menon <avmenon@codeaurora.org>