Linux 5.12 update: bio_max_segs() replaces BIO_MAX_PAGES

The BIO_MAX_PAGES macro is being retired in favor of a bio_max_segs()
function that implements the typical MIN(x,y) logic used throughout the
kernel for bounding the allocation, and also the new implementation is
intended to be signed-safe (which the former was not).

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Coleman Kane <ckane@colemankane.org>
Closes #11765
(cherry picked from commit ffd6978ef5)
Signed-off-by: Jonathon Fernyhough <jonathon@m2x.dev>

config/kernel-bio_max_segs.m4

@@ -0,0 +1,23 @@
+dnl #
+dnl # 5.12 API change removes BIO_MAX_PAGES in favor of bio_max_segs()
+dnl # which will handle the logic of setting the upper-bound to a
+dnl # BIO_MAX_PAGES, internally.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_BIO_MAX_SEGS], [
+	ZFS_LINUX_TEST_SRC([bio_max_segs], [
+		#include <linux/bio.h>
+	],[
+		bio_max_segs(1);
+	])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_BIO_MAX_SEGS], [
+	AC_MSG_CHECKING([whether bio_max_segs() exists])
+	ZFS_LINUX_TEST_RESULT([bio_max_segs], [
+		AC_MSG_RESULT(yes)
+
+		AC_DEFINE([HAVE_BIO_MAX_SEGS], 1, [bio_max_segs() is implemented])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -127,6 +127,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_GENERIC_FILLATTR_USERNS
 	ZFS_AC_KERNEL_SRC_MKNOD
 	ZFS_AC_KERNEL_SRC_SYMLINK
+	ZFS_AC_KERNEL_SRC_BIO_MAX_SEGS
 
 	AC_MSG_CHECKING([for available kernel interfaces])
 	ZFS_LINUX_TEST_COMPILE_ALL([kabi])
@@ -227,6 +228,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_GENERIC_FILLATTR_USERNS
 	ZFS_AC_KERNEL_MKNOD
 	ZFS_AC_KERNEL_SYMLINK
+	ZFS_AC_KERNEL_BIO_MAX_SEGS
 ])
 
 dnl #