test/librbd/fsx: wait for resize to propagate in krbd_resize()

test/librbd/fsx: wait for resize to propagate in krbd_resize()

With this changes resize request will not be blocked until the resize is
completed. Because of this the fsx test fails as it assumes that the
request to resize immediately implies changes on the device size.

Hence we have to add a wait in resize handler of fsx for the device to
actually get resized.

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>