afs: Do better max capacity handling on address lists

afs: Do better max capacity handling on address lists

Note the maximum allocated capacity in an afs_addr_list struct and discard
addresses that would exceed it in afs_merge_fs_addr{4,6}().

Also, since the current maximum capacity is less than 255, reduce the
relevant members to bytes.

Signed-off-by: David Howells <dhowells@redhat.com>