OPENAFS-SA-2018-002 afs: prevent RXAFSCB_TellMeAboutYourself information leak

OPENAFS-SA-2018-002 afs: prevent RXAFSCB_TellMeAboutYourself information leak

RXAFSCB_TellMeAboutYourself does not completely initialize its output
buffers.  This leaks kernel memory over the wire:

struct interfaceAddr
Unix cache manager (libafs)
- up to 124 bytes in array addr_in ((AFS_MAX_INTERFACE_ADDR 32 * 4) - 4))
- up to 124 bytes in array subnetmask	"
- up to 124 bytes in array mtu		"

Windows cache manager
- 64 bytes in array addr_in ((AFS_MAX_INTERFACE_ADDR 32 - CM_MAXINTERFACE_ADDR 16)* 4)
- 64 bytes in array subnetmask	"
- 64 bytes in array mtu         "

The following implementations of SRXAFSCB_TellMeAboutYourself are not susceptible:
- fsprobe
- libafscp
- xstat_fs_test

Initialize the buffer.

Change-Id: I2ef868dd9269db7004a21cf913b6787948357d10