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>

arch/powerpc/xmon/ppc-opc.c

@@ -1587,7 +1587,7 @@ extract_tbr (unsigned long insn,
 #define CTX(op, xop)   (OP (op) | (((unsigned long)(xop)) & 0x7))
 #define CTX_MASK CTX(0x3f, 0x7)
 
-/* An User Context form instruction.  */
+/* A User Context form instruction.  */
 #define UCTX(op, xop)  (OP (op) | (((unsigned long)(xop)) & 0x1f))
 #define UCTX_MASK UCTX(0x3f, 0x1f)