scripts/spelling.txt: add "an user" pattern and fix typo instances

Fix typos and add the following to the scripts/spelling.txt:

  an user||a user
  an userspace||a userspace

I also added "userspace" to the list since it is a common word in Linux.
I found some instances for "an userfaultfd", but I did not add it to the
list.  I felt it is endless to find words that start with "user" such as
"userland" etc., so must draw a line somewhere.

Link: http://lkml.kernel.org/r/1481573103-11329-4-git-send-email-yamada.masahiro@socionext.com
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/arch/Kconfig b/arch/Kconfig
index d0012ad..cd211a1 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -29,7 +29,7 @@
 	  The number of hardware counters is limited. The multiplexing
 	  feature enables OProfile to gather more events than counters
 	  are provided by the hardware. This is realized by switching
-	  between events at an user specified time interval.
+	  between events at a user specified time interval.
 
 	  If unsure, say N.