afs: Fix getting the afs.fid xattr

afs: Fix getting the afs.fid xattr

The AFS3 FID is three 32-bit unsigned numbers and is represented as three
up-to-8-hex-digit numbers separated by colons to the afs.fid xattr.
However, with the advent of support for YFS, the FID is now a 64-bit volume
number, a 96-bit vnode/inode number and a 32-bit uniquifier (as before).
Whilst the sprintf in afs_xattr_get_fid() has been partially updated (it
currently ignores the upper 32 bits of the 96-bit vnode number), the size
of the stack-based buffer has not been increased to match, thereby allowing
stack corruption to occur.

Fix this by increasing the buffer size appropriately and conditionally
including the upper part of the vnode number if it is non-zero.  The latter
requires the lower part to be zero-padded if the upper part is non-zero.

Fixes: 41bbaa1b38d8 ("afs: Increase to 64-bit volume ID and 96-bit vnode ID for YFS")
Signed-off-by: David Howells <dhowells@redhat.com>