xfs: make xfs_can_free_eofblocks a predicate

The two patches in this set reorganize the responsibilities between
xfs_can_free_eofblocks and xfs_free_eofblocks so that the first becomes
a true predicate, and the second becomes a simple update function.  The
goal is to be able to use the predicate to decide if a linked but
unopened inode has speculative post-EOF preallocations and hence must go
through the extra inactivation step.

This requires a slight change in behavior of the background block gc
workers, which will try to take the IOLOCK before calling the predicate.