fscache: Fix if condition in fscache_wait_on_volume_collision()

After waiting for the volume to complete the acquisition with timeout,
the if condition under which potential volume collision occurs should be
acquire the volume is still pending rather than not pending so that we
will continue to wait until the pending flag is cleared. Also, use the
existing test pending wrapper directly instead of test_bit().

Fixes: 62ab6335

 ("fscache: Implement volume registration")
Signed-off-by: Yue Hu <huyue2@coolpad.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://listman.redhat.com/archives/linux-cachefs/2022-May/006918.html