Kernel 4.20 allows writes & reads up to 1 MiB (before: 128 kiB)
via CAP_MAX_PAGES & MaxPages.

Instead of exposing MaxPages in the API, we follow what libfuse
does, and calculate MaxPages from MaxWrite (rounding up).

Contrary to what libfuse does, we also set max_read to the same
value as MaxWrite. This prevents reads getting larger than writes
due to the rounding-up for MaxPages, which is unexpected. This
also changes the default behavoir of go-fuse, which was 64 kiB
writes, but 128 kiB for reads. Now it is 128 kiB for both.

The tests are implemented in the fs package because it's
easier there. They also test MaxReadAhead.

Tested on Linux 4.19.0 and Linux 6.1.7 via all.bash,
and on 6.1.7 also via the gocryptfs test suite.

Supersedes https://github.com/hanwen/go-fuse/pull/347

Change-Id: I5a1d4ee91945155c367888da7a90814a24a9ee6e