sysfs: add sysfs_open_file_mutex

Add a separate mutex to protect sysfs_open_dirent->buffers list.  This
will allow performing sleepable operations while traversing
sysfs_buffers, which will be renamed to sysfs_open_file.

Note that currently sysfs_open_dirent->buffers list isn't being used
for anything and this patch doesn't make any functional difference.
It will be used to merge regular and bin file supports.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>