debugfs: Silence DEBUG_STRICT_USER_COPY_CHECKS=y warning

Enabling DEBUG_STRICT_USER_COPY_CHECKS causes the following
warning:

In file included from arch/x86/include/asm/uaccess.h:573,
                 from include/linux/uaccess.h:5,
                 from include/linux/highmem.h:7,
                 from include/linux/pagemap.h:10,
                 from fs/debugfs/file.c:18:
In function 'copy_from_user',
    inlined from 'write_file_bool' at fs/debugfs/file.c:435:
arch/x86/include/asm/uaccess_64.h:65: warning: call to
'copy_from_user_overflow' declared with attribute warning:
copy_from_user() buffer size is not provably correct

presumably due to buf_size being signed causing GCC to fail to
see that buf_size can't become negative.
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

fs/debugfs/file.c

@@ -428,7 +428,7 @@ static ssize_t write_file_bool(struct file *file, const char __user *user_buf,
 			       size_t count, loff_t *ppos)
 {
 	char buf[32];
-	int buf_size;
+	size_t buf_size;
 	bool bv;
 	u32 *val = file->private_data;