common/btrfs: lookup running processes using pgrep

common/btrfs: lookup running processes using pgrep

Certain helper functions and the testcase btrfs/132 use the following
script to find running processes:

	while ps aux | grep "balance start" | grep -qv grep; do
	<>
	done

Instead, using pgrep is more efficient.

	while pgrep -f "btrfs balance start" > /dev/null; do
	<>
	done

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>