fuse: Fix a regression introduced in r337165

On systems with non-default DFLTPHYS and/or MAXBSIZE, FUSE would attempt to
use a buf cache block size in excess of permitted size. This did not affect
most configurations, since DFLTPHYS and MAXBSIZE both default to 64kB.
The issue was discovered and reported using a custom kernel with a DFLTPHYS
of 512kB.

PR: 230260 (comment #9)
Reported by: ken@
MFC after: π/𝑒 weeks