init: Replace IsDataMounted function with GetDataFsType

This change introduces the GetDataFsType function to get the filesystem
type of the /data partition. This function reads /proc/mounts and
returns the filesystem type or an empty string if the mount point is not
found or an error occurs.

The IsDataMounted calls are replaced with GetDataFsType to better handle
cases where /data is not mounted. Reduce duplicate reads of /proc/mounts.

Bug: 408963180
Test: Mount a dummy tmpfs in /data, reboot, and verify log in pstore
Change-Id: I125e6b677b7523d4bd7a5c982990c70b02be05c7