fs: Move clearing of mappedtodisk to buffer.c

fs: Move clearing of mappedtodisk to buffer.c

The mappedtodisk flag is only meaningful for buffer head based
filesystems.  It should not be cleared for other filesystems.  This allows
us to reuse the mappedtodisk flag to have other meanings in filesystems
that do not use buffer heads.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20241002040111.1023018-2-willy@infradead.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>