libblkid: read() optimization for small devices

libblkid: read() optimization for small devices

 - don't read the begin (69kB) of the device by one large read()
 - fill in the SB buffer dynamically
 - use extra buffer for FATs root dir entries (FAT FS label)
   on small devices to avoid large reads

Detect FAT12 on 1.4MB device (number of bytes):

Old version:

  $ strace -e read blkid -p floppy.img 2>&1 | \
	awk -F ' = ' 'BEGIN {x=0} /read/ && !/.*ELF/ {x += $2} END { print x }'
  73292

New version:
  $ strace -e read blkid -p floppy.img 2>&1 | \
	awk -F ' = ' 'BEGIN {x=0} /read/ && !/.*ELF/ {x += $2} END { print x }'
  8192

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Karel Zak <kzak@redhat.com>