xfs: reduce ilock roundtrips in xfs_qm_vop_dqalloc

xfs: reduce ilock roundtrips in xfs_qm_vop_dqalloc

xfs_qm_vop_dqalloc only needs the (exclusive) ilock for attaching dquots
to the inode if not done so yet.  All the other locks don't touch the inode
and don't need the ilock - the i_rwsem / iolock protects against changes
to the IDs while we are in a method, and the ilock would not help because
dropping it for the dqget calls would be racy anyway.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>