sh: Fix unneeded constructor in page table allocation

The pgd kmem_cache allocation both specified __GFP_ZERO and had a
constructor which makes no sense.  Remove __GFP_ZERO and zero the user
parts of the pgd explicitly.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Rich Felker <dalias@libc.org>