Avoid LUN ID collisions in NetApp iSCSI drivers

Avoid LUN ID collisions in NetApp iSCSI drivers

Previously, the logic to calculate the LUN ID only considered a single attached
host. Because multiple hosts can be attached to a LUN, the logic needs to
consider multiple hosts. This patchset proposes to choose non-conflicting
LUN IDs across the whole array. This patch first checks for an empty LUN ID
across the array. If it doesn't find one, then it looks for the least used
LUN ID across the whole array.
The value of MAX_LUNS_PER_HOST has been changed here as the range function
iterates from 0 to n-1 LUN IDs. With Eseries backend, a maximum of 256 LUN_IDS
are supported, thus a value of 256 MAX_LUNS_PER_HOST is required.

Closes-bug: 1433825
Closes-bug: 1457995
Change-Id: I033368c12c90d7fbbaf81b065f1a1c8c5975006a