fuse: limit user-specified values of max background requests

An untrusted user could DoS the system if s/he were allowed to accumulate an
arbitrary number of pending background requests by setting the above limits
to extremely high values in INIT. This patch excludes this possibility by
imposing global upper limits on the possible values of per-mount "max
background requests" and "congestion threshold" parameters for unprivileged
FUSE filesystems.

These global limits are implemented as module parameters.

Signed-off-by: Csaba Henk <csaba@gluster.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>