staging: android: ashmem: fix a race condition in ASHMEM_SET_SIZE ioctl

A lock-unlock is missing in ASHMEM_SET_SIZE ioctl which can result in a
race condition when mmap is called. After the !asma->file check, before
setting asma->size, asma->file can be set in mmap. That would result in
having different asma->size than the mapped memory size. Combined with
ASHMEM_UNPIN ioctl and shrinker invocation, this can result in memory
corruption.

Bug: 66954097
Signed-off-by: Viktor Slavkovic <viktors@google.com>
Change-Id: I268225133f96fde0fadd1ec621aafef27d392d65
Signed-off-by: Satyanarayana Dash <sadash@codeaurora.org>