Fix a potential sleep while holding a mutex in the sa(4) driver.

If the user issues a MTIOCEXTGET ioctl, and the tape drive in question has
a serial number that is longer than 80 characters, we malloc a buffer in
saextget() to hold the output of cam_strvis().

Since a mutex is held in that codepath, doing a M_WAITOK malloc could lead
to sleeping while holding a mutex. Change it to a M_NOWAIT malloc and bail
out if we fail to allocate the memory. Devices with serial numbers longer
than 80 bytes are very rare (I don't recall seeing one), so this
should be a very unusual case to hit. But it is a bug that should be fixed.

sys/cam/scsi/scsi_sa.c:
In saextget(), if we need to malloc a buffer to hold the output of
cam_strvis(), don't wait for the memory. Fail and return an error
if we can't allocate the memory immediately.

PR: kern/220094
Submitted by: Jia-Ju Bai <baijiaju1990@163.com>
MFC after: 3 days
Sponsored by: Spectra Logic