ext4: Use page_symlink() instead of __page_symlink()

By using the memalloc_nofs_save() functionality, we can call
page_symlink(), safe in the knowledge that it won't recurse into the
filesystem.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>