lib/kstrtox.c: add "false"/"true" support to kstrtobool()

At many places in kernel, It is necessary to convert sysfs input to
corresponding bool value e.g.  "false" or "0" need to be converted to bool
false, "true" or "1" need to be converted to bool true, places where such
conversion is needed currently check the input string manually,
kstrtobool() can be utilized at such places but currently it doesn't have
support to accept "false"/"true".

Add support to accept "false"/"true" as valid string in kstrtobool().

[akpm@linux-foundation.org: undo s/iff/if/, per Matthew]
Link: https://lkml.kernel.org/r/20220426180203.70782-1-jvgediya@linux.ibm.com
Signed-off-by: Jagdish Gediya <jvgediya@linux.ibm.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Richard Fitzgerald <rf@opensource.cirrus.com>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>