btrfs: avoid taking the device_list_mutex in btrfs_run_dev_stats()

btrfs: avoid taking the device_list_mutex in btrfs_run_dev_stats()

btrfs_run_dev_stats() is called during the critical section of a
transaction commit and it takes the device_list_mutex, which is also
acquired by fitrim, which does discard operations while holding that
mutex. Most of the time, if we are on a healthy filesystem, we don't have
new stat updates to persist in the device tree, so blocking on the
device_list_mutex is just wasting time and making any tasks that need to
start a new transaction wait longer that necessary.

Since the device list is RCU safe/protected, make btrfs_run_dev_stats()
do an initial check for device stat updates using RCU and quit without
taking the device_list_mutex in case there are no new device stats that
need to be persisted in the device tree.

Also note that adding/removing devices also requires starting a
transaction, and since btrfs_run_dev_stats() is called from the critical
section of a transaction commit, no one can be concurrently adding or
removing a device while btrfs_run_dev_stats() is called.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>