Make CONFIG_SLOW_WORK an automatic rather than manual config option

Make CONFIG_SLOW_WORK an automatic rather than manual config option so that
people configuring their kernels don't have to make the choice.  It can be
selected automatically by those things that require it (such as FS-Cache).

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Jeff Garzik <jgarzik@redhat.com>
Acked-by: Kyle McMartin <kyle@mcmartin.ca>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>