libxcmd: fix mount option parsing to find rt/log devices

libxcmd: fix mount option parsing to find rt/log devices

It turns out that glibc's hasmntopt implementation returns NULL
if the opt parameter ends with an equals ('=').  Therefore, we
cannot directly search for the option 'rtdev='; we must instead
have hasmntopt look for 'rtdev' and look for the trailing equals
sign ourselves.  This fixes xfs_info's reporting of external
log and realtime device paths, and xfs_scrub will need it for
data block scrubbing of realtime extents.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>