btrfs: convert ioctl handlers to AUTO_KFREE

btrfs: convert ioctl handlers to AUTO_KFREE

Many ioctl handlers are suitable for the AUTO_KFREE conversions as the
data are temporary and short lived. The conversions are trivial or the
collateral changes are straightforward.

A kfree() preceding mnt_drop_write_file() is slightly more efficient but
in the reverse order (i.e. the automatic kfree) does not cause any
significant change as the write drop does only a few simple operations.

Note: __free() handles also error pointers, so this is safe for the
memdup_user() errors too.

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