LU-13982 tests: fix infinite loop in sanity test_184c

LU-13982 tests: fix infinite loop in sanity test_184c

If the dd in test_184c fails to create the file, for example due to
ENOSPC, the subsequent "while" loops indefinitely.

So add a loop-count to ensure it stops eventually.

Also change the test from "-f" to "-s" so we wait for the file to be
non-empty.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I5051adf101f08856b97fa994f687b976fda84df4
Reviewed-on: https://review.whamcloud.com/40007
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>