fusefs: fix the tests for a wider range of maxphys

maxphys is now a tunable, ever since r368124. The default value is also
larger than it used to be. That broke several fusefs tests that made
assumptions about maxphys.

• WriteCluster.clustering used the MAXPHYS compile-time constant.

• WriteBackAsync.direct_io_partially_overlaps_cached_block implicitly depended on the default value of maxphys. Fix it by making the dependency explicit.

• Write.write_large implicitly assumed that maxphys would be no more than twice maxbcachebuf. Fix it by explicitly setting m_max_write.

• WriteCluster.clustering and several others failed because the MockFS module did not work for max_write > 128KB (which most tests would set when maxphys > 256KB). Limit max_write accordingly. This is the same as fusefs-libs's behavior.

• Bmap's tests were originally written for MAXPHYS=128KB. With larger values, the simulated file size was too small.

PR: 252096
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D27769