libata: disable a disk via libata.force params

commit b8bd6dc3 upstream.

A user on StackExchange had a failing SSD that's soldered directly
onto the motherboard of his system. The BIOS does not give any option
to disable it at all, so he can't just hide it from the OS via the
BIOS.

The old IDE layer had hdX=noprobe override for situations like this,
but that was never ported to the libata layer.

This patch implements a disable flag for libata.force.

Example use:

 libata.force=2.0:disable

[v2 of the patch, removed the nodisable flag per Tejun Heo]
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Link: http://unix.stackexchange.com/questions/102648/how-to-tell-linux-kernel-3-0-to-completely-ignore-a-failing-disk
Link: http://askubuntu.com/questions/352836/how-can-i-tell-linux-kernel-to-completely-ignore-a-disk-as-if-it-was-not-even-co
Link: http://superuser.com/questions/599333/how-to-disable-kernel-probing-for-drive
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

Documentation/kernel-parameters.txt

@@ -1305,6 +1305,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 			* dump_id: dump IDENTIFY data.
 
+			* disable: Disable this device.
+
 			If there are multiple matching configurations changing
 			the same attribute, the last one is used.
 

drivers/ata/libata-core.c

@@ -6331,6 +6331,7 @@ static int __init ata_parse_force_one(char **cur,
 		{ "nohrst",	.lflags		= ATA_LFLAG_NO_HRST },
 		{ "nosrst",	.lflags		= ATA_LFLAG_NO_SRST },
 		{ "norst",	.lflags		= ATA_LFLAG_NO_HRST | ATA_LFLAG_NO_SRST },
+		{ "disable",	.horkage_on	= ATA_HORKAGE_DISABLE },
 	};
 	char *start = *cur, *p = *cur;
 	char *id, *val, *endp;