fat: Add KUnit tests for checksums and timestamps

Add some basic sanity-check tests for the fat_checksum() function and
the fat_time_unix2fat() and fat_time_fat2unix() functions. These unit
tests verify these functions return correct output for a number of test
inputs.

These tests were inspired by -- and serve a similar purpose to -- the
timestamp parsing KUnit tests in ext4[1].

Note that, unlike fat_time_unix2fat, fat_time_fat2unix wasn't previously
exported, so this patch exports it as well. This is required for the
case where we're building the fat and fat_test as modules.

Fixed minor checkpatch coding style errors and typos in commit log:
Shuah Khan <skhan@linuxfoundation.org>

[1]:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/ext4/inode-test.c

Signed-off-by: David Gow <davidgow@google.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
diff --git a/fs/fat/Kconfig b/fs/fat/Kconfig
index 66532a7..238cc55 100644
--- a/fs/fat/Kconfig
+++ b/fs/fat/Kconfig
@@ -77,7 +77,7 @@
 
 config FAT_DEFAULT_CODEPAGE
 	int "Default codepage for FAT"
-	depends on MSDOS_FS || VFAT_FS
+	depends on FAT_FS
 	default 437
 	help
 	  This option should be set to the codepage of your FAT filesystems.
@@ -115,3 +115,15 @@
 	  Say Y if you use UTF-8 encoding for file names, N otherwise.
 
 	  See <file:Documentation/filesystems/vfat.rst> for more information.
+
+config FAT_KUNIT_TEST
+	tristate "Unit Tests for FAT filesystems" if !KUNIT_ALL_TESTS
+	depends on KUNIT && FAT_FS
+	default KUNIT_ALL_TESTS
+	help
+	  This builds the FAT KUnit tests
+
+	  For more information on KUnit and unit tests in general, please refer
+	  to the KUnit documentation in Documentation/dev-tools/kunit
+
+	  If unsure, say N