[SCSI] sd: tell the user when a disk's capacity is adjusted

This patch (as1188) combines the tests for decrementing a drive's
reported capacity and expands the comment.  It also adds an
informational message to the system log, informing the user when the
reported value has been changed.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>