xfs: prevent deadlock in xfs_qm_shake()

It's possible to recurse into filesystem from the memory
allocation, which deadlocks in xfs_qm_shake(). Add check
for __GFP_FS, and bail out if it is not set.

Signed-off-by: Felix Blyakher <felixb@sgi.com>
Signed-off-by: Hedi Berriche <hedi@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>