GFS2: Merge two nearly identical xattr functions

There were two functions in the xattr code which were nearly
identical, the only difference being that one was copy data into
the unstuffed xattrs and the other was copying data out from it.

This patch merges the two functions such that the code which deal
with iteration over the unstuffed xattrs is no longer duplicated.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>