btrfs: rename btrfs_folio_(set|start|end)_writer_lock()

[ Upstream commit 0f712026 ]

Since there is no user of reader locks, rename the writer locks into a
more generic name, by removing the "_writer" part from the name.

And also rename btrfs_subpage::writer into btrfs_subpage::locked.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: 8bf334be ("btrfs: fix double accounting race when extent_writepage_io() failed")
Signed-off-by: Sasha Levin <sashal@kernel.org>