[SCSI] ch: fix device minor number management bug

ch_probe uses the total number of ch devices as minor.

ch_probe:
	ch->minor = ch_devcount;
...
	ch_devcount++;

Then ch_remove decreases ch_devcount:

ch_remove:
	ch_devcount--;

If you have two ch devices, sch0 and sch1, and remove sch0,
ch_devcount is 1. Then if you add another ch device, ch_probe tries to
create sch1. So you get a warning and fail to create sch1:

Jan 24 16:01:05 nice kernel: sysfs: duplicate filename 'sch1' can not be created
Jan 24 16:01:05 nice kernel: WARNING: at fs/sysfs/dir.c:424 sysfs_add_one()
Jan 24 16:01:05 nice kernel: Pid: 2571, comm: iscsid Not tainted 2.6.24-rc7-ga3d2c2e8-dirty #1
Jan 24 16:01:05 nice kernel:
Jan 24 16:01:05 nice kernel: Call Trace:
Jan 24 16:01:05 nice kernel:  [<ffffffff802a22b8>] sysfs_add_one+0x54/0xbd
Jan 24 16:01:05 nice kernel:  [<ffffffff802a283c>] create_dir+0x4f/0x87
Jan 24 16:01:05 nice kernel:  [<ffffffff802a28a9>] sysfs_create_dir+0x35/0x4a
Jan 24 16:01:05 nice kernel:  [<ffffffff803069a1>] kobject_get+0x12/0x17
Jan 24 16:01:05 nice kernel:  [<ffffffff80306ece>] kobject_add+0xf3/0x1a6
Jan 24 16:01:05 nice kernel:  [<ffffffff8034252b>] class_device_add+0xaa/0x39d
Jan 24 16:01:05 nice kernel:  [<ffffffff803428fb>] class_device_create+0xcb/0xfa
Jan 24 16:01:05 nice kernel:  [<ffffffff80229e09>] printk+0x4e/0x56
Jan 24 16:01:05 nice kernel:  [<ffffffff802a2054>] sysfs_ilookup_test+0x0/0xf
Jan 24 16:01:05 nice kernel:  [<ffffffff88022580>] :ch:ch_probe+0xbe/0x61a

(snip)

This patch converts ch to use a standard minor number management way,
idr like sg and bsg.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>