afs: Concoct ctimes

The in-kernel afs filesystem ignores ctime because the AFS fileserver
protocol doesn't support ctimes.  This, however, causes various xfstests to
fail.

Work around this by:

 (1) Setting ctime to attr->ia_ctime in afs_setattr().

 (2) Not ignoring ATTR_MTIME_SET, ATTR_TIMES_SET and ATTR_TOUCH settings.

 (3) Setting the ctime from the server mtime when on the target file when
     creating a hard link to it.

 (4) Setting the ctime on directories from their revised mtimes when
     renaming/moving a file.

Found by the generic/221 and generic/309 xfstests.

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