tracing: Add missing syscall_metadata comment

Add the missing syscall_metadata description for the enter_fields
struct member.

Link: http://lkml.kernel.org/r/74c3407cd1e5d37f2c5aaca637aa4d35f66f1aa2.1372479499.git.tom.zanussi@linux.intel.com



Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>