fs/proc: re-factor proc_pid_cmdline_read() a bit

This is a pure refactoring of the function, preparing for some further
cleanups.  The thing was pretty illegible, and the core functionality
still is, but now the core loop is a bit more isolated from the thing
that goes on around it.

This was "inspired" by the confluence of kworker workqueue name cleanups
by Tejun, currently scheduled for 4.18, and commit 7f7ccc2c ("proc:
do not access cmdline nor environ from file-backed areas").

Change-Id: Ia50851c44fce78de4eb4e6f7b2ef5b91cdda74c1
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Git-commit: e4b4e441
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Prateek Sood <prsood@codeaurora.org>