fuse_lowlevel: Add max_pages support (#384)

fuse_lowlevel: Add max_pages support (#384)

Starting with kernel version 4.20 fuse supports a new property
'max_pages' which is the maximum number of pages that can be used per
request. This can be set via an argument during initialization.
This new property allows writes to be larger than 128k.

This patch sets the property if the matching capability is set
(FUSE_MAX_PAGES). It will also set max_write to 1MiB. Filesystems have
the possibility to decrease this size by setting max_write to a smaller
size. The max_pages and bufsize fields are adjusted accordingly.

Cc: Constantine Shulyupin <const@MakeLinux.com>
Signed-off-by: Markus Pargmann <scosu@quobyte.com>