ptrace: Remove maxargs from task_current_syscall()

ptrace: Remove maxargs from task_current_syscall()

task_current_syscall() has a single user that passes in 6 for maxargs, which
is the maximum arguments that can be used to get system calls from
syscall_get_arguments(). Instead of passing in a number of arguments to
grab, just get 6 arguments. The args argument even specifies that it's an
array of 6 items.

This will also allow changing syscall_get_arguments() to not get a variable
number of arguments, but always grab 6.

Linus also suggested not passing in a bunch of arguments to
task_current_syscall() but to instead pass in a pointer to a structure, and
just fill the structure. struct seccomp_data has almost all the parameters
that is needed except for the stack pointer (sp). As seccomp_data is part of
uapi, and I'm afraid to change it, a new structure was created
"syscall_info", which includes seccomp_data and adds the "sp" field.

Link: http://lkml.kernel.org/r/[email protected]
Cc: Andy Lutomirski <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Al Viro <[email protected]>
Cc: [email protected]
Reviewed-by: Thomas Gleixner <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>