Btrfs: fix a compiling warning

Btrfs: fix a compiling warning

NOTE:
This is against btrfs-next git repo.

fs/btrfs/inode.c: In function ‘relink_extent_backref’:
fs/btrfs/inode.c:2185:6: warning: passing argument 2 of
‘btrfs_drop_extents’ from incompatible pointer type [enabled by default]
fs/btrfs/ctree.h:3340:5: note: expected ‘struct btrfs_root *’ but argument is of type ‘struct inode *’
fs/btrfs/inode.c:2185:6: warning: passing argument 3 of ‘btrfs_drop_extents’ makes pointer from integer without a cast [enabled by default]
fs/btrfs/ctree.h:3340:5: note: expected ‘struct inode *’ but argument is of type ‘u64’
fs/btrfs/inode.c:2185:6: warning: passing argument 5 of ‘btrfs_drop_extents’ makes integer from pointer without a cast [enabled by default]
fs/btrfs/ctree.h:3340:5: note: expected ‘u64’ but argument is of type ‘u64 *’

We didn't update the related functions after someone changed the API.

This fixes it.

Reported-by: <fengguang.wu@intel.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>