FS-Cache: Allow the current state of all objects to be dumped

Allow the current state of all fscache objects to be dumped by doing:

	cat /proc/fs/fscache/objects

By default, all objects and all fields will be shown.  This can be restricted
by adding a suitable key to one of the caller's keyrings (such as the session
keyring):

	keyctl add user fscache:objlist "<restrictions>" @s

The <restrictions> are:

	K	Show hexdump of object key (don't show if not given)
	A	Show hexdump of object aux data (don't show if not given)

And paired restrictions:

	C	Show objects that have a cookie
	c	Show objects that don't have a cookie
	B	Show objects that are busy
	b	Show objects that aren't busy
	W	Show objects that have pending writes
	w	Show objects that don't have pending writes
	R	Show objects that have outstanding reads
	r	Show objects that don't have outstanding reads
	S	Show objects that have slow work queued
	s	Show objects that don't have slow work queued

If neither side of a restriction pair is given, then both are implied.  For
example:

	keyctl add user fscache:objlist KB @s

shows objects that are busy, and lists their object keys, but does not dump
their auxiliary data.  It also implies "CcWwRrSs", but as 'B' is given, 'b' is
not implied.

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