fstests: generic/352 should accomodate other pwrite behaviors

fstests: generic/352 should accomodate other pwrite behaviors

xfs_io pwrite issues a series of block size writes, but there is no
guarantee that the resulting extent(s) will be singular or contiguous.
This behavior is acceptable, but the test is flawed in that it expects
a single extent for a pwrite.

Modify test to limit pwrite and reflink to a single block.

Signed-off-by: Bill O'Donnell <bodonnel@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>