Btrfs: send: fix error number for unknown inode types

ENOTSUPP should not be returned to the user program.
 (cf. include/linux/errno.h)
Therefore, EOPNOTSUPP is used instead of ENOTSUPP.

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>