xen/blkback: do not BUG() for invalid blkif_request from frontend

xen/blkback: do not BUG() for invalid blkif_request from frontend

Upstream commit 0e367ae46503 ("xen/blkback: correctly respond to unknown,
non-native requests") fixed a bug to correctly respond to unknown,
non-native requests,  e.g., BLKIF_OP_RESERVED_1 or BLKIF_OP_PACKET for
64-bit SLES 11 guests when using a 32-bit backend.

Although such fix is already in uek4, it is broken by commit f0af2f840606
("xen-blkback: move indirect req allocation out-of-line") that introduced
the BUG() again.

This patch removes the BUG() to avoid panic backend by invalid
blkif_request from frontend.

Orabug: 29199843

Fixes: f0af2f840606 ("xen-blkback: move indirect req allocation out-of-line")
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Joe Jin <joe.jin@oracle.com>
Reviewed-by: Ankur Arora <ankur.a.arora@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>