ecryptfs: Use a folio in ecryptfs_write_begin()

ecryptfs: Use a folio in ecryptfs_write_begin()

Use __filemap_get_folio() instead of grab_cache_page_write_begin()
and use the folio throughout.  No attempt is made here to support
large folios, simply converting this function to use folio APIs is
the goal.  Saves many hidden calls to compound_head().

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>