btrfs: free pages on error in btrfs_uring_read_extent()

In the Linux kernel, the following vulnerability has been resolved:

btrfs: free pages on error in btrfs_uring_read_extent()

In this function the 'pages' object is never freed in the hopes that it is

picked up by btrfs_uring_read_finished() whenever that executes in the

future. But that's just the happy path. Along the way previous

allocations might have gone wrong, or we might not get -EIOCBQUEUED from

btrfs_encoded_read_regular_fill_pages(). In all these cases, we go to a

cleanup section that frees all memory allocated by this function without

assuming any deferred execution, and this also needs to happen for the

'pages' allocation.