blk: fix wrong idr_pre_get() error check in loop.c

The idr_pre_get() function never returns a value < 0.  It returns 0 (no
memory) or 1 (OK).

Reported-by: Silva Paulo <psdasilva@yahoo.com>
[ Rewrote Silva's patch, but attributing it to Silva anyway  - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>