losetup: don't clear FD at EBUSY error in loopcxt_setup_device()

loopback lib clears the existing FD unconditionally at error in
loopcxt_setup_device().  This is done even after EBUSY, thus the second call
actually clears the previous setup wrongly.

Author:  Takashi Iwai <tiwai@suse.de>
Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>
losetup: don't ignore EBUSY in losetup when the loop device is given

losetup ignores the EBUSY error.  In the loop fo A_CREATE action, it skips
the error handling when errno = EBUSY.  It's OK for the case where no loop
device is specified, but the error has to be handled when the device is given
explicitly.

Author:  Takashi Iwai <tiwai@suse.de>
Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>