xfs: reduce indirect function calls

This short series reduces the number of indirect function calls when we
want to iterate the incore inode radix tree, and reduces the number of
arguments that must be passed to the walk function.

I made a few observations about incore inode radix tree walks -- the one
caller (blockgc) that cares about radix tree tags is internal to
xfs_icache.c, and there's a 1:1 mapping between that tag and the
iterator function.  Furthermore, the only other caller (quotaoff) is the
only caller to supply a nonzero flags argument, and it never specifies a
radix tree tag.

On those grounds, we can remove both the flags and tag arguments to
xfs_inode_walk; and for the internal caller, we can replace the indirect
call with direct calls to the blockgc functions.  This makes for less
ugly code and gives us a (negligible) performance bump.

This series is a prerequisite for the next patchset, since deferred
inode inactivation will add another inode radix tree tag and iterator
function to go with it.