kernel: make groups_sort calling a responsibility group_info allocators

kernel: make groups_sort calling a responsibility group_info allocators

In testing, we found that nfsd threads may call set_groups in parallel
for the same entry cached in auth.unix.gid, racing in the call of
groups_sort, corrupting the groups for that entry and leading to
permission denials for the client.

This patch:
 - Make groups_sort globally visible.
 - Move the call to groups_sort to the modifiers of group_info
 - Remove the call to groups_sort from set_groups

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thiago Rafael Becker <[email protected]>
Reviewed-by: Matthew Wilcox <[email protected]>
Reviewed-by: NeilBrown <[email protected]>
Acked-by: "J. Bruce Fields" <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>