Don't try to mount if the disk has no FS magic

This avoids:
[   22.932529][    T1] init: [libfs_mgr]Invalid f2fs superblock on '/dev/block/platform/14700000.ufs/by-name/metadata'
[   22.934609][    T1] F2FS-fs (sda8): Magic Mismatch, valid(0xf2f52010) - read(0x0)
[   22.935061][    T1] F2FS-fs (sda8): Can't find valid F2FS filesystem in 1th superblock
[   22.937306][    T1] F2FS-fs (sda8): Magic Mismatch, valid(0xf2f52010) - read(0x0)
[   22.943700][    T1] F2FS-fs (sda8): Can't find valid F2FS filesystem in 2th superblock
[   22.951937][    T1] init: [libfs_mgr]__mount(source=/dev/block/platform/14700000.ufs/by-name/metadata,target=/metadata,type=f2fs)=-1: Invalid argument

Bug: 210589189
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Change-Id: I27989b25769eae83eb06ac86146f27baf288b7e1