xfs: make xfs_inode_set_eofblocks_tag cheaper for the common case

For long growing file writes we will usually already have the
eofblocks tag set when adding more speculative preallocations.  Add
a flag in the inode to allow us to skip the the fairly expensive
AG-wide spinlocks and multiple radix tree operations in that case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>