vfs_gpfs: Avoid calling gpfs_is_offline on every i/o

vfs_gpfs: Avoid calling gpfs_is_offline on every i/o

Asks gpfs as long as a file is offline. Once it was reported online once,
we'll not ask anymore.  This assumes that while we have a file open it
won't be migrated away. This might not *always* be true, but probably
close enough. And as long as we don't have a proper notification mechanism
and as long as polling is too expensive, this seems like a good strategy.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Christof Schmitt <cs@samba.org>
Autobuild-User(master): Christof Schmitt <cs@samba.org>
Autobuild-Date(master): Sat Sep  5 01:50:09 CEST 2015 on sn-devel-104