init: fix first stage mount failure due to /dev/device-mapper not found

It has been reported that fs_mgr failed to open /dev/device-mapper
during the first stage mount. It's because other uevent (e.g., i2c
charger device) happens to be sent at the same time we're triggering
the device-mapper uevent to be sent. Current implementation returns
COLDBOOT_STOP unconditionally so it will only process the first received
uevent, leaving device-mapper uevent unhandled when the race happens.

Fix this by only returning COLDBOOT_STOP when the received uevent->path
matches that of device mapper.

Bug: 37745254

Test: first stage mount /vendor with vboot 2.0 (avb) on bullhead
Test: first stage mount /vendor with vboot 1.0 on sailfish
Change-Id: I4a77093ec8f90a5ca981a088f34d082d0270533b