staging: android: ashmem: Don't call fallocate() with ashmem_mutex held.

syzbot is hitting lockdep warnings [1][2][3]. This patch tries to fix
the warning by eliminating ashmem_shrink_scan() => {shmem|vfs}_fallocate()
sequence.

[1] https://syzkaller.appspot.com/bug?id=87c399f6fa6955006080b24142e2ce7680295ad4
[2] https://syzkaller.appspot.com/bug?id=7ebea492de7521048355fc84210220e1038a7908
[3] https://syzkaller.appspot.com/bug?id=e02419c12131c24e2a957ea050c2ab6dcbbc3270



Reported-by: syzbot <syzbot+a76129f18c89f3e2ddd4@syzkaller.appspotmail.com>
Reported-by: syzbot <syzbot+148c2885d71194f18d28@syzkaller.appspotmail.com>
Reported-by: syzbot <syzbot+4b8b031b89e6b96c4b2e@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: stable@vger.kernel.org
Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>