Medium: Filesystem: improve read/write checks for CHECK_LEVEL 10, 20

Some devices don't support 512 byte direct writes anymore,
the "physical" block size is 4kByte.

Changing "bs=512 count=16" to "bs=4k count=1" also helps to avoid
spurious timeouts. I've seen badly tuned (and/or totally overloaded) IO
stacks exposing a synchronous single-request IO latency of many seconds
(under load). If we need 16 of those, we can easily violate timeout
settings.