drivers/char/mem: implement splice() for /dev/zero, /dev/full

This allows splicing zeroed pages into a pipe, and allows discarding
pages from a pipe by splicing them to /dev/zero. Writing to /dev/zero
should have the same effect as writing to /dev/null, and a
"splice_write" implementation exists only for /dev/null.

(The /dev/zero splice_read implementation could be optimized by
pushing references to the global zero page to the pipe, but that's an
optimization for another day.)

Signed-off-by: Max Kellermann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/char/mem.c

···
 #define full_lseek      null_lseek
 #define write_zero	write_null
 #define write_iter_zero	write_iter_null
+#define splice_write_zero	splice_write_null
 #define open_mem	open_port
 
 static const struct file_operations __maybe_unused mem_fops = {
···
 	.read_iter	= read_iter_zero,
 	.read		= read_zero,
 	.write_iter	= write_iter_zero,
+	.splice_read	= copy_splice_read,
+	.splice_write	= splice_write_zero,
 	.mmap		= mmap_zero,
 	.get_unmapped_area = get_unmapped_area_zero,
 #ifndef CONFIG_MMU
···
 	.llseek		= full_lseek,
 	.read_iter	= read_iter_zero,
 	.write		= write_full,
+	.splice_read	= copy_splice_read,
 };
 
 static const struct memdev {