staging: lustre: introduce and use l_wait_event_abortable()

lustre sometimes wants to wait for an event, but abort if
one of a specific list of signals arrives.  This is a little
bit like wait_event_killable(), except that the signals are
identified a different way.

So introduce l_wait_event_abortable() which provides this
functionality.
Having separate functions for separate needs is more in line
with the pattern set by include/linux/wait.h, than having a
single function which tries to include all possible needs.

Also introduce l_wait_event_abortable_exclusive().

Note that l_wait_event() return -EINTR on a signal, while
Linux wait_event functions return -ERESTARTSYS.
l_wait_event_{abortable_,}exclusive follow the Linux pattern.

Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>