lib: proportions.c trivial sparse lock annotation

Suppresses sparse warning:
lib/proportions.c:159:16: warning: context imbalance in 'prop_get_global': wrong count at exit
lib/proportions.c:159:16:    context 'RCU': wanted 0, got 1
lib/proportions.c:164:2: warning: context imbalance in 'prop_put_global': unexpected unlock
lib/proportions.c:164:2:    context 'RCU': wanted 0, got -1

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>