btrfs: only run the extent map shrinker from kswapd tasks

Currently the extent map shrinker can be run by any task when attempting
to allocate memory and there's enough memory pressure to trigger it.

To avoid too much latency we stop iterating over extent maps and removing
them once the task needs to reschedule. This logic was introduced in commit
b3ebb9b7 ("btrfs: stop extent map shrinker if reschedule is needed").

While that solved high latency problems for some use cases, it's still
not enough because with a too high number of tasks entering the extent map
shrinker code, either due to memory allocations or because they are a
kswapd task, we end up having a very high level of contention on some
spin locks, namely:

1) The fs_info->fs_roots_radix_lock spin lock, which we need to find
   roots to iterate over their inodes;

2) The spin lock of the xarray used to track open inodes for a root
   (struct btrfs_root::inodes) - on 6.10 kernels and below, it used to
   be a red black tree and the spin lock was root->inode_lock;

3) The fs_info->delayed_iput_lock spin lock since the shrinker adds
   delayed iputs (calls btrfs_add_delayed_iput()).

Instead of allowing the extent map shrinker to be run by any task, make
it run only by kswapd tasks. This still solves the problem of running
into OOM situations due to an unbounded extent map creation, which is
simple to trigger by direct IO writes, as described in the changelog
of commit 956a17d9 ("btrfs: add a shrinker for extent maps"), and
by a similar case when doing buffered IO on files with a very large
number of holes (keeping the file open and creating many holes, whose
extent maps are only released when the file is closed).

Reported-by: kzd <kzd@56709.net>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=219121


Reported-by: Octavia Togami <octavia.togami@gmail.com>
Link: https://lore.kernel.org/linux-btrfs/CAHPNGSSt-a4ZZWrtJdVyYnJFscFjP9S7rMcvEMaNSpR556DdLA@mail.gmail.com/


Fixes: 956a17d9 ("btrfs: add a shrinker for extent maps")
CC: stable@vger.kernel.org # 6.10+
Tested-by: kzd <kzd@56709.net>
Tested-by: Octavia Togami <octavia.togami@gmail.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>