btrfs: remove inode_lock from struct btrfs_root and use xarray locks

Currently we use the spinlock inode_lock from struct btrfs_root to
serialize access to two different data structures:

1) The delayed inodes xarray (struct btrfs_root::delayed_nodes);
2) The inodes xarray (struct btrfs_root::inodes).

Instead of using our own lock, we can use the spinlock that is part of the
xarray implementation, by using the xa_lock() and xa_unlock() APIs and
using the xarray APIs with the double underscore prefix that don't take
the xarray locks and assume the caller is using xa_lock() and xa_unlock().

So remove the spinlock inode_lock from struct btrfs_root and use the
corresponding xarray locks. This brings 2 benefits:

1) We reduce the size of struct btrfs_root, from 1336 bytes down to
   1328 bytes on a 64 bits release kernel config;

2) We reduce lock contention by not using anymore the same lock for
   changing two different and unrelated xarrays.

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