Currently, shrink_slab() will decide that it needs to scan a certain number of
dentries, will call shrink_dcache_memory() requesting that this be done, and
shrink_dcache_memory() will simply bale out without doing anything because the
caller did not have __GFP_FS.

This has the potential to disrupt our lovely pagecache-vs-slab balancing act. 
So change things so that shrinker callouts can return -1, indicating that they
baled out.  This way, shrink_slab can remember that this slab was owed a
certain number of scannings and these will be correctly performed next time a
__GFP_FS caller comes by.