Introduce a helper exported from lib/pdu.c which cancels all pdus for a
given context. This patch eliminates repeated code from various other
files which have the same purpose. The only functional difference is
that the cancellation done from iscsi-command.c was (incorrectly) not
checking for iscsi->is_loggedin before issuing callbacks.
Signed-off-by: Felipe Franciosi <felipe@nutanix.com>