xfs: Update inode alloc comments

I found some out of date comments while studying the inode allocation
code, so I believe it's worth to have these comments updated.

It basically rewrites the comment regarding to "call_again" variable,
which is not used anymore, but instead, callers of xfs_ialloc() decides
if it needs to be called again relying only if ialloc_context is NULL or
not.

Also did some small changes in another comment that I thought to be
pertinent to the current behaviour of these functions and some alignment
on both comments.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>