xfs: do not allocate the entire delalloc extent in xfs_bmapi_write

While trying to convert the entire delalloc extent is a good decision
for regular writeback as it leads to larger contigous on-disk extents,
but for other callers of xfs_bmapi_write is is rather questionable as
it forced them to loop creating new transactions just in case there
is no large enough contiguous extent to cover the whole delalloc
reservation.

Change xfs_bmapi_write to only allocate the passed in range instead,
whіle the writeback path through xfs_bmapi_convert_delalloc and
xfs_bmapi_allocate still always converts the full extents.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>