subsys/fs/littlefs: fix Coverity issues

Closes #18392 by asserting and returning an error if the block size is
not positive.

Closes #18458.  The diagnosis here was not relevant as an in-range EOS
is written before the buffer is used, but using the non-terminated
length is slightly more clear about intent and may avoid a read overrun
of the mount point.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>