ALSA: pcmtest: Return -EFAULT on pattern read copy failure

ALSA: pcmtest: Return -EFAULT on pattern read copy failure

pattern_write() reports -EFAULT when copy_from_user() fails, but
pattern_read() converts copy_to_user() failures into a zero-length read.
That makes a userspace buffer fault look like EOF instead of reporting the
actual error.

Return -EFAULT from pattern_read() when copying the pattern data to
userspace fails, and update the file offset only after a successful copy.

Fixes: 315a3d57c64c ("ALSA: Implement the new Virtual PCM Test Driver")
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260501-alsa-pcmtest-pattern-read-efault-v1-1-53e1e8c11dda@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>