Do not automatically "select" SCSI_DH for dm-multipath. If SCSI_DH
doesn't exist,just do not allow  hardware handlers to be used.

Handle SCSI_DH being a module also. Make sure it doesn't allow DM_MULTIPATH
to be compiled in when SCSI_DH is a module.

[jejb: added comment for Kconfig syntax]
Signed-off-by: NChandra Seetharaman <sekharan@us.ibm.com>
Reported-by: NRandy Dunlap <randy.dunlap@oracle.com>
Reported-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NJames Bottomley <James.Bottomley@HansenPartnership.com>