[ia64] ptrace_[sg]etregs(): use access_elf_reg() instead of access_uarea()

In case of positions passed by ptrace_[sg]etregs() to access_uarea()
the latter sets the stack unwind up, walks all the way up the stack
and proceeds to pass the resulting info to access_elf_reg().  The thing
is, we'd *already* obtained that info, so we can bloody well call
access_elf_reg() directly.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>