vfs: stop polls before calling close()

After close() is called, the file should not be used any more. We need
to stop pollers before doing that, otherwise epoll_wait() can attempt
calling poll() on a file which was closed. Doing so will result in
aborts like the one below, because the socket, which is attempted to
be locked has been already freed.

[backtrace]
0x00000000002232a8 <__assert_fail+24>
0x00000000003a7e27 <lockfree::mutex::unlock()+487>
0x00000000003dfe0d <epoll_file::wait(epoll_event*, int, int)+1165>
0x00000000003de240 <epoll_wait+112>
0x0000100004409ad2 <Java_sun_nio_ch_EPollArrayWrapper_epollWait+210>

Fixes #424.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>