fsck: allow stat access on /dev/block files

To determine if it is safe to run fsck on a filesystem, e2fsck
must first determine if the filesystem is mounted. To do that,
e2fsck scans through /proc/mounts and collects all the mounted
block devices. With that information, it runs stat() on each block
device, comparing the major and minor numbers to the filesystem
passed in on the command line. If there is a match, then the filesystem
is currently mounted and running fsck is dangerous.
Allow stat access to all block devices so that fsck can compare
major/minor values.

Addresses the following denials:

avc: denied { getattr } for comm="e2fsck" path="/dev/block/sde5"
dev="tmpfs" ino=15649 scontext=u:r:fsck:s0
tcontext=u:object_r:metadata_block_device:s0 tclass=blk_file
permissive=0

avc: denied { getattr } for comm="e2fsck" path="/dev/block/sda25"
dev="tmpfs" ino=15528 scontext=u:r:fsck:s0
tcontext=u:object_r:modem_block_device:s0 tclass=blk_file permissive=0

avc: denied { getattr } for comm="e2fsck" path="/dev/block/sda31"
dev="tmpfs" ino=15552 scontext=u:r:fsck:s0
tcontext=u:object_r:system_block_device:s0 tclass=blk_file permissive=0

avc: denied { getattr } for comm="e2fsck" path="/dev/block/sdd3"
dev="tmpfs" ino=15600 scontext=u:r:fsck:s0
tcontext=u:object_r:block_device:s0 tclass=blk_file permissive=0

Bug: 35324014
Bug: 33781554
Test: device boots and no SELinux denials.
Change-Id: I5af4a334ec41952887914eec4eee5c60cc441a66