Fix fstat()/lseek() use in MDXFileChunk()

[jtl]

Summary

As I understand it, @bde voiced three concerns:

1. Improper use of fstat()/lseek().
2. Improper documentation of the return value for the function.
3. Speed due to I/O buffer sizes.

This addresses concern #1.

Test Plan

make && make test

Created a md and was able to use md5 on it:

# mdconfig -a -t swap -s 64m
md0
# env LD_PRELOAD=/usr/obj/usr/src/lib/libmd/libmd.so md5 /dev/md0
MD5 (/dev/md0) = 462be07247e06c953b885b168d1753a1
# sh -c 'for i in /dev/md0; do echo -n "$i: "; md5 <$i; done'
/dev/md0: 462be07247e06c953b885b168d1753a1

[jtl]

Updating with changes based on review by bde@.

I believe he now has accepted these changes. I plan to commit them soon unless I hear further feedback.

[allanjude]

Seems reasonable to me

[delphij]

Looks good to me.

[jtl]

Committed in rS294037.