Various components restrict size of IO passed up to the userspace filesystem
based on the mount's f_iosize value. The previous default of PAGE_SIZE
is anemic, even for normal filesystems, but especially considering every
FUSE operation involves a kernel <-> userspace IPC upcall.
Bump to DFLTPHYS (currently 64kB) to match other FUSE implementations.
Anecdotally, Jakub reports IO read performance increased from 600 MB/s ->
2700 MB/s with a basic RAM-backed FUSE filesystem.
PR: 230260
Reported by: Peter (MooseFS) <freebsd AT moosefs.com>
Tested by: Jakub Kruszona-Zawadzki <acid AT moosefs.com>
MFC after: 3 days
dab6195cd3