proc: mandate ->proc_lseek in "struct proc_ops"

Now that proc_ops are separate from file_operations and other operations
it easy to check all instances to have ->proc_lseek hook and remove check
in main code.

Note:
nonseekable_open() files naturally don't require ->proc_lseek.

Garbage collect pde_lseek() function.

[adobriyan@gmail.com: smoke test lseek()]
  Link: https://lkml.kernel.org/r/YG4OIhChOrVTPgdN@localhost.localdomain

Link: https://lkml.kernel.org/r/YFYX0Bzwxlc7aBa/@localhost.localdomain
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/proc/read.c

@@ -14,7 +14,7 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 // Test
-// 1) read of every file in /proc
+// 1) read and lseek on every file in /proc
 // 2) readlink of every symlink in /proc
 // 3) recursively (1) + (2) for every directory in /proc
 // 4) write to /proc/*/clear_refs and /proc/*/task/*/clear_refs
@@ -45,6 +45,8 @@ static void f_reg(DIR *d, const char *filename)
 	fd = openat(dirfd(d), filename, O_RDONLY|O_NONBLOCK);
 	if (fd == -1)
 		return;
+	/* struct proc_ops::proc_lseek is mandatory if file is seekable. */
+	(void)lseek(fd, 0, SEEK_SET);
 	rv = read(fd, buf, sizeof(buf));
 	assert((0 <= rv && rv <= sizeof(buf)) || rv == -1);
 	close(fd);