Make vfs_resize_fd_table() accessible in the kernel

Also update some types from int to uint32.
Increase the size of the kernel FD table

With packagefs potentially opening quite a few packages the default of
256 slots is a bit tight. It's 4096 now, which should be safe for a
while, but we might want to consider resizing the table dynamically and
probably even switching to another algorithm for allocating the slots.

Should fix #11328.