When reading a btree node (or leaf), at read_block_for_search(), if we
can't find its extent buffer in the cache (the fs_info->buffer_radix
radix tree), then we unlock all upper level nodes before reading the
btree node/leaf from disk, to prevent blocking other tasks for too long.

However if we find that the extent buffer is in the cache but it is not
up to date, we don't unlock upper level nodes before reading it from disk,
potentially blocking other tasks on upper level nodes for too long.

Fix this inconsistent behaviour by unlocking upper level nodes if we need
to read a node/leaf from disk because its in-memory extent buffer is not
up to date. If we unlocked upper level nodes then we must return -EAGAIN
to the caller, just like the case where the extent buffer is not cached in
memory. And like that case, we determine if upper level nodes are locked
by checking only if the parent node is locked - if it isn't, then no other
upper level nodes are locked.

This is actually a rare case, as if we have an extent buffer in memory,
it typically has the uptodate flag set and passes all the checks done by
btrfs_buffer_uptodate().
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>