client/fuse: set max_idle_threads to the correct value

client/fuse: set max_idle_threads to the correct value

When the version of the libfuse is 3.1 or later,
the fuse worker thread of ceph-fuse keeps recreating
and deleting file or directory will be blocked forever.
It is caused by parameter 'max_idle_threads' being set to 0
and the relevant logic is in function 'fuse_do_work' of libfuse.
Parameter 'max_idle_threads' can be set by function 'fuse_parse_cmdline',
it may be the default value(10 before version 3.12 and -1 after)
or a value set by user. It should not be set to 0.

Fixes: https://tracker.ceph.com/issues/58109
Signed-off-by: Zhansong Gao <zhsgao@hotmail.com>