io_uring: fix min types mismatch in table alloc

fs/io_uring.c: In function 'io_alloc_page_table':
include/linux/minmax.h:20:28: warning: comparison of distinct pointer
	types lacks a cast

Cast everything to size_t using min_t.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 9123c8ff

 ("io_uring: add helpers for 2 level table alloc")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/50f420a956bca070a43810d4a805293ed54f39d8.1623759527.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>