selftests: gpio-mockup-chardev: Check asprintf() for error

[ Upstream commit 508cacd7da6659ae7b7bdd0a335f675422277758 ]

With gcc 7.3.0:

    gpio-mockup-chardev.c: In function ‘get_debugfs’:
    gpio-mockup-chardev.c:62:3: warning: ignoring return value of ‘asprintf’, declared with attribute warn_unused_result [-Wunused-result]
       asprintf(path, "%s/gpio", mnt_fs_get_target(fs));
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Handle asprintf() failures to fix this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>