xfs: abort fstrim if kernel is suspending

A recent ext4 patch posting from Jan Kara reminded me of a
discussion a year ago about fstrim in progress preventing kernels
from suspending. The fix is simple, we should do the same for XFS.

This removes the -ERESTARTSYS error return from this code, replacing
it with either the last error seen or the number of blocks
successfully trimmed up to the point where we detected the stop
condition.

References: https://bugzilla.kernel.org/show_bug.cgi?id=216322
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>