As SCSI SBC4 specification section 5.30.2 describes that it can
support unmapping one or more LBA range in single UNMAP command.
However, previously we only pack one LBA range in UNMAP command
by default no matter device gives the block limits that says it
can support unmapping multiple LBA ranges with a single UNMAP
command.
This patch sets max_discard_segments config according to block
limits of device, and supports unmapping multiple LBA ranges with
a single UNMAP command.