btrfs: reuse existing inode from btrfs_ioctl

btrfs_ioctl extracts inode from file so we can pass that into the
callbacks.

Signed-off-by: Sahil Kang <sahil.kang@asilaycomputing.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>