xfs: Compute bmap extent alignments in a separate function

This commit moves over the code which computes stripe alignment and
extent size hint alignment into a separate function. Apart from
xfs_bmap_btalloc(), the new function will be used by another function
introduced in a future commit.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>