filemap: Convert generic_perform_write() to support large folios

Modelled after the loop in iomap_write_iter(), copy larger chunks from
userspace if the filesystem has created large folios.

[hch: use mapping_max_folio_size to keep supporting file systems that do
 not support large folios]

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>