kernfs: Drop unnecessary "mutex" local variable initialization

kernfs: Drop unnecessary "mutex" local variable initialization

These are unnecessary and unconventional. Remove them. Also move variable
declaration into the block that it's used. No functional changes.

Cc: Imran Khan <imran.f.khan@oracle.com>
Tested-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220828050440.734579-3-tj@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>