sysfs: count subdirectories

This patch introduces a subdirectory counter for each sysfs directory.

Without the patch, sysfs_refresh_inode would walk all entries of the directory
to calculate the number of subdirectories.

This patch improves time of "ls -la /sys/block" when there are 10000 block
devices from 9 seconds to 0.19 seconds.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>