flex_array: export symbols to modules

Alex said:

  I want to use flex_array to store a sparse array of ATM cell
  re-assembly buffers for my ATM over Ethernet driver.  Using the per-vcc
  user_back structure causes problems when stacked with things like
  br2684.

Add EXPORT_SYMBOL() for all publically accessible flex array functions
and move to obj-y so that modules may use this library.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Reported-by: Alex Bennee <kernel-hacker@bennee.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>