MFC r182600: In rev. 1.17 (r33548) of msdosfs_fat.c, relative cluster numbers were replaced by file relative sector numbers as the buffer block number when zero-padding a file during extension. Revert the change, it causes wrong blocks filled with zeroes on seeking beyond end of file.
MFC r182600:
In rev. 1.17 (r33548) of msdosfs_fat.c, relative cluster numbers were
replaced by file relative sector numbers as the buffer block number
when zero-padding a file during extension. Revert the change, it causes
wrong blocks filled with zeroes on seeking beyond end of file.

PR:	kern/47628
Approved by:	re (kensmith)

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