xfs: clean up incore inode walk functions

This 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, it never specifies a
radix tree tag, and it only walks inodes that have VFS state.

The first patch moves quotaoff to walk the vfs inode list to drop all
attached dquots, which frees us to remove the iter_flags argument and
the indirect function calls from xfs_inode_walk.

Next, we merge the code that walks reclaimable inodes into
xfs_inode_walk and refactor all the code that sets and clears the radix
tree tags in the perag structure and the perag tree itself.

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.

v2: walk the vfs inode list when running quotaoff instead of the radix
    tree, then rework the (now completely internal) inode walk function
    to take the tag as the main parameter.
v3: merge the reclaim loop into xfs_inode_walk, then consolidate the
    radix tree tagging functions