[XFS] kill xfs_igrow_start and xfs_igrow_finish

xfs_igrow_start just expands to xfs_zero_eof with two asserts that are
useless in the context of the only caller and some rather confusing
comments.

xfs_igrow_finish is just a few lines of code decorated again with useless
asserts and confusing comments.

Just kill those two and merge them into xfs_setattr.

SGI-PV: 981498
SGI-Modid: xfs-linux-melb:xfs-kern:31186a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>