lib: os: mpsc_pbuf: do not wait when spinlock is held

Check if the spinlock is held before attempting to wait by
taking the semaphore, as that would cause a context switch which
isn't allowed and will trigger an assertion error when
`CONFIG_SPIN_VALIDATE` is enabled.

Logging in spinlock-held context when the log buffer is full can lead
to an infinite assertion error loop, as the logging subsys attempts to
allocate buffer when there's none available, it will try to wait for
one and thus triggers the assertion error, the error message will be
printed through the logging sybsys but there's no buffer available,
so it will try to wait for one and triggers another assertion error..
This loop just goes on and on forever, and nothing gets printed to
the terminal.

Added a test to validate the fix.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>
Signed-off-by: Maxim Adelman <imax@meta.com>