shmem: set a_ops earlier in shmem_symlink

Set the a_ops in shmem_symlink before reading a folio from the mapping
to prepare for asserting that shmem_get_folio is only called on shmem
mappings.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>