[SCSI] target: Avoid mem leak and needless work in transport_generic_get_mem

In drivers/target/target_core_transport.c::transport_generic_get_mem()
there are a few potential memory leaks in the error paths. This patch
makes sure that we free previously allocated memory when other allocations
fail.  It also moves some work (INIT_LIST_HEAD() and assignment to
se_mem->se_len) below all the allocations so that if something fails we
don't do the work at all.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>