target/rd: Refactor rd_build_device_space + rd_release_device_space

This patch refactors rd_build_device_space() + rd_release_device_space()
into rd_allocate_sgl_table() + rd_release_device_space() so that they
may be used seperatly for setup + release of protection information
scatterlists.

Also add explicit memset of pages within rd_allocate_sgl_table() based
upon passed 'init_payload' value.

v2 changes:
  - Drop unused sg_table from rd_release_device_space (Wei)

Change-Id: I0d42113c0e2d703fe7567a804003b034fd4fa0b7
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Git-commit:4442dc8a92b8f9ad8ee9e7f8438f4c04c03a22dc
Git-repo:https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git
[ggober@codeaurora.org:resolve trivial merge conflicts in target_core_rd.c]
Signed-off-by: Gopal G Goberu <ggober@codeaurora.org>