ceph-fuse: Improve fuse mount usage message

Show '--client_fs <fsname>' option in 'ceph-fuse' usage
which is required to mount a specific fs in multifs cluster.

https://tracker.ceph.com/issues/67283
Signed-off-by: Kotresh HR <[email protected]>

Author: kotreshhr

src/ceph_fuse.cc

@@ -81,9 +81,10 @@ static void fuse_usage()
 void usage()
 {
   cout <<
-"usage: ceph-fuse [-n client.username] [-m mon-ip-addr:mon-port] <mount point> [OPTIONS]\n"
-"  --client_mountpoint/-r <sub_directory>\n"
-"                    use sub_directory as the mounted root, rather than the full Ceph tree.\n"
+"\nusage: ceph-fuse [-n client.username] [-m mon-ip-addr:mon-port] [--client_fs <fsname>] [--client_mountpoint/-r <sub_directory>] <mount point> [OPTIONS]\n\n"
+
+"  --client_mountpoint/-r: use sub_directory as the mounted root, rather than the full CephFS tree.\n"
+"  --client_fs: named file system to mount (default: usually the first file system created).\n"
 "\n";
   fuse_usage();
   generic_client_usage();