xfs: push the ilock into xfs_zero_eof

Instead of calling xfs_zero_eof with the ilock held only take it internally
for the minimall required critical section around xfs_bmapi_read.  This
also requires changing the calling convention for xfs_zero_last_block
slightly.  The actual zeroing operation is still serialized by the iolock,
which must be taken exclusively over the call to xfs_zero_eof.

We could in fact use a shared lock for the xfs_bmapi_read calls as long as
the extent list has been read in, but given that we already hold the iolock
exclusively there is little reason to micro optimize this further.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>