init: snapuserd: Fix ranges for mlock()

It cannot be assumed that file mappings in /proc/<pid>/maps will be
completely backed by the underlying file. [1]

Use MappedFileSize() to deduce the correct ranges for the mlock()
calls when locking system pages in the OTA path.

While at it also clean up the some unreachable code (mlockall()),
and improve error logging.

[1] SIGBUS at https://man7.org/linux/man-pages/man2/mmap.2.html#RETURN_VALUE



Test: Incremental OTA
Bug: 324952273
Change-Id: Ia2ab150e1b8de8c638f5b1acc1de83deb7ac5cff
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>