flex_array: add helpers to get and put to make pointers easy to use

Getting and putting arrays of pointers with flex arrays is a PITA.  You
have to remember to pass &ptr to the _put and you have to do weird and
wacky casting to get the ptr back from the _get.  Add two functions
flex_array_get_ptr() and flex_array_put_ptr() to handle all of the magic.

[akpm@linux-foundation.org: simplification suggested by Joe]
Signed-off-by: Eric Paris <eparis@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Joe Perches <joe@perches.com>
Cc: James Morris <jmorris@namei.org>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>