btrfs: extent-tree: Make btrfs_inode_rsv_refill function static

This function is no longer used outside of extent-tree.c.
Make it static.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>