btrfs: reuse existing pointers from btrfs_ioctl

btrfs_ioctl already contains pointers to the inode and btrfs_root
structs, so we can pass them into the subfunctions instead of the
toplevel struct file.

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