btrfs: bring back the incorrectly removed extent buffer lock recursion support

Commit 51b03b74 ("btrfs: locking: remove the recursion handling code")
from the 5.10.233 stable tree removed the support for extent buffer lock
recursion, but we need that code because in 5.10.x we load the free space
cache synchronously - while modifying the extent tree and holding a write
lock on some extent buffer, we may need to load the free space cache,
which requires acquiring read locks on the extent tree and therefore
result in a deadlock in case we need to read lock an extent buffer we had
write locked while modifying the extent tree.

Backporting that commit from Linus' tree is therefore wrong, and was done
so in order to backport upstream commit 97e86631 ("btrfs: don't set
lock_owner when locking extent buffer for reading"). However we should
have instead had the commit adapted to the 5.10 stable tree instead.

Note that the backport of that dependency is ok only for stable trees
5.11+, because in those tree the space cache loading code is not
synchronous anymore, so there is no need to have the lock recursion
and indeed there are no users of the extent buffer lock recursion
support. In other words, the backport is only valid for kernel releases
that have the asynchrounous free space cache loading support, which
was introduced in kernel 5.11 with commit e747853c ("btrfs: load
free space cache asynchronously").

This was causing deadlocks and reported by a user (see below Link tag).

So revert commit 51b03b74 ("btrfs: locking: remove the recursion
handling code") while not undoing what commit d5a30a61 ("btrfs: don't
set lock_owner when locking extent buffer for reading") from the 5.10.x
stable tree did.

Reported-by: pk <pkoroau@gmail.com>
Link: https://lore.kernel.org/linux-btrfs/CAMNwjEKH6znTHE5hMc5er2dFs5ypw4Szx6TMDMb0H76yFq5DGQ@mail.gmail.com/


Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>