fuse_lowlevel FUSE_INIT: Simplify the max_write/bufsize logic

fuse_lowlevel FUSE_INIT: Simplify the max_write/bufsize logic

max_write can be limited by se->op.init() and by the buffer size,
we use the minimum of these two.
Required se->bufsize is then set according to the determined
max_write. The current thread will have the old buffer size,
though, as it already had to the allocation to handle the
FUSE_INIT call (unless splice is used and ths variable
and related buffer is not used at all).

The given bufsize is just a hint for minimum size, allocation
could be actually larger (for example to get huge pages).