file-posix: Include filename in locking error message

file-posix: Include filename in locking error message

Image locking errors happening at device initialization time doesn't say
which file cannot be locked, for instance,

    -device scsi-disk,drive=drive-1: Failed to get shared "write" lock
    Is another process using the image?

could refer to either the overlay image or its backing image.

Hoist the error_append_hint to the caller of raw_check_lock_bytes where
file name is known, and include it in the error hint.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>