generic/692: generalize the test for non-4K Merkle tree block sizes

generic/692: generalize the test for non-4K Merkle tree block sizes

Due to the assumption of the Merkle tree block size being 4K, the file
size calculated for the second test case was taking way too long to hit
EFBIG in case of larger block sizes like 64K.  Fix this by generalizing
the calculation to any Merkle tree block size >= 1K.

Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Co-developed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>