procfs: Clean up huge if-statement in __proc_file_read()

Switch huge if-statement in __proc_file_read() around.  This then puts the
single line loop break immediately after the if-statement and allows us to
de-indent the huge comment and make it take fewer lines.  The code following
the if-statement then follows naturally from the call to dp->read_proc().

Signed-off-by: David Howells <dhowells@redhat.com>