ceph: make ioctl cmds more readable in debug log

    ioctl file 0000000004e6b054 cmd 2148296211 arg 824635143532

The numerical cmd value in the ioctl debug log message is too hard to
understand even when you look at it in the code. Make it more readable.

[ idryomov: add missing _ in ceph_ioctl_cmd_name() ]

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-and-tested-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>