xfs: use an empty transaction for fstrim

xfs: use an empty transaction for fstrim

We currently use a btree walk in the fstrim code. This requires a
btree cursor and btree cursors are only used inside transactions
except for the fstrim code. This means that all the btree operations
that allocate memory operate in both GFP_KERNEL and GFP_NOFS
contexts.

This causes problems with lockdep being unable to determine the
difference between objects that are safe to lock both above and
below memory reclaim. Free space btree buffers are definitely locked
both above and below reclaim and that means we have to mark all
btree infrastructure allocations with GFP_NOFS to avoid potential
lockdep false positives.

If we wrap this btree walk in an empty cursor, all btree walks are
now done under transaction context and so all allocations inherit
GFP_NOFS context from the tranaction. This enables us to move all
the btree allocations to GFP_KERNEL context and hence help remove
the explicit use of GFP_NOFS in XFS.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>