[XFS] Fix speculative allocation beyond eof

Speculative allocation beyond eof doesn't work properly.  It was
broken some time ago after a code cleanup that moved what is now
xfs_iomap_eof_align_last_fsb() and xfs_iomap_eof_want_preallocate()
out of xfs_iomap_write_delay() into separate functions.  The code
used to use the current file size in various checks but got changed
to be max(file_size, i_new_size).  Since i_new_size is the result
of 'offset + count' then in xfs_iomap_eof_want_preallocate() the
check for '(offset + count) <= isize' will always be true.

ie if 'offset + count' is > ip->i_size then isize will be i_new_size
and equal to 'offset + count'.

This change fixes all the places that used to use the current file
size.

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