new helper: single_open_size()

Same as single_open(), but preallocates the buffer of given size.
Doesn't make any sense for sizes up to PAGE_SIZE and doesn't make
sense if output of show() exceeds PAGE_SIZE only rarely - seq_read()
will take care of growing the buffer and redoing show().  If you
_know_ that it will be large, it might make more sense to look into
saner iterator, rather than go with single-shot one.  If that's
impossible, single_open_size() might be for you.

Again, don't use that without a good reason; occasionally that's really
the best way to go, but very often there are better solutions.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
fs/seq_file: fallback to vmalloc allocation
fs/seq_file: Use vmalloc by default for allocations > PAGE_SIZE

Some OOM implementations are pretty trigger happy when it comes to
releasing memory for kmalloc() allocations.  We might as well head
straight to vmalloc for allocations over PAGE_SIZE.

Change-Id: Ic0dedbadc8bf551d34cc5d77c8073938d4adef80
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
nick kvfree() from apparmor