mips: call find_vma with the mmap_sem held

mips: call find_vma with the mmap_sem held

Performing vma lookups without taking the mm->mmap_sem is asking for
trouble.  While doing the search, the vma in question can be modified or
even removed before returning to the caller.  Take the lock (exclusively)
in order to avoid races while iterating through the vmacache and/or
rbtree.

Updates two functions:
  - process_fpemu_return()
  - cteon_flush_cache_sigtramp()

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Tested-by: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>