Fix signed/unsigned integer comparison warning

allowed_pending_fence_count is apparently an int32_t, which made clang
unhappy when compared with the size_t from calling size().

Cast size() to an int32_t, since I doubt we'll have > 2.1 billion FDs
open any time soon. :)

Bug: None
Test: mma. Warning is gone.
Change-Id: I895b2c6e1a6d5a42c692470f6a5c5fd22d55b2ff