netfs: Implement buffered writes through netfs_file_write_iter()

Institute a netfs write helper, netfs_file_write_iter(), to be pointed at
by the network filesystem ->write_iter() call.  Make it handled buffered
writes by copying the data into the pagecache and creating a dirty regions
to represent contiguous spans of compatible data.

Dirty regions are considered compatible, and thus mergeable, if they are
part of the same flush group, have the same fscache requirements and have
the same O_SYNC/O_DSYNC characteristics - plus any constraints the netfs
wishes to add.

Signed-off-by: David Howells <dhowells@redhat.com>

fs/netfs/Makefile

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 
 netfs-y := \
+	buffered_write.o \
 	direct.o \
 	misc.o \
 	objects.o \