KEYS: Use individual pages in big_key for crypto buffers

kmalloc() can't always allocate large enough buffers for big_key to use for
crypto (1MB + some metadata) so we cannot use that to allocate the buffer.
Further, vmalloc'd pages can't be passed to sg_init_one() and the aead
crypto accessors cannot be called progressively and must be passed all the
data in one go (which means we can't pass the data in one block at a time).

Fix this by allocating the buffer pages individually and passing them
through a multientry scatterlist to the crypto layer.  This has the bonus
advantage that we don't have to allocate a contiguous series of pages.

We then vmap() the page list and pass that through to the VFS read/write
routines.

This can trigger a warning:

	WARNING: CPU: 0 PID: 60912 at mm/page_alloc.c:3883 __alloc_pages_nodemask+0xb7c/0x15f8
	([<00000000002acbb6>] __alloc_pages_nodemask+0x1ee/0x15f8)
	 [<00000000002dd356>] kmalloc_order+0x46/0x90
	 [<00000000002dd3e0>] kmalloc_order_trace+0x40/0x1f8
	 [<0000000000326a10>] __kmalloc+0x430/0x4c0
	 [<00000000004343e4>] big_key_preparse+0x7c/0x210
	 [<000000000042c040>] key_create_or_update+0x128/0x420
	 [<000000000042e52c>] SyS_add_key+0x124/0x220
	 [<00000000007bba2c>] system_call+0xc4/0x2b0

from the keyctl/padd/useradd test of the keyutils testsuite on s390x.

Note that it might be better to shovel data through in page-sized lumps
instead as there's no particular need to use a monolithic buffer unless the
kernel itself wants to access the data.

Fixes: 13100a72 ("Security: Keys: Big keys stored encrypted")
Reported-by: Paul Bunyan <pbunyan@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Kirill Marinushkin <k.marinushkin@gmail.com>