ext4: support uncached buffered I/O

ext4: support uncached buffered I/O

Set FOP_DONTCACHE in ext4_file_operations to declare support for
uncached buffered I/O.

To handle this flag, update ext4_write_begin() and ext4_da_write_begin()
to use write_begin_get_folio(), which encapsulates FGP_DONTCACHE logic
based on iocb->ki_flags.

Part of a series refactoring address_space_operations write_begin and
write_end callbacks to use struct kiocb for passing write context and
flags.

Signed-off-by: Taotao Chen <chentaotao@didiglobal.com>
Link: https://lore.kernel.org/20250716093559.217344-6-chentaotao@didiglobal.com
Signed-off-by: Christian Brauner <brauner@kernel.org>