GFS2: use __vmalloc GFP_NOFS for fs-related allocations.

leaf_dealloc uses vzalloc as a fallback to kzalloc(GFP_NOFS), so
it clearly does not want any shrinker activity within the fs itself.
convert vzalloc into __vmalloc(GFP_NOFS|__GFP_ZERO) to better achieve
this goal.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>