xfs: fix xfs_init_attr_trans not handling explicit operation codes

When we were converting the attr code to use an explicit operation code
instead of keying off of attr->value being null, we forgot to change the
code that initializes the transaction reservation.  Split the function
into two helpers that handle the !remove and remove cases, then fix both
callsites to handle this correctly.

Fixes: c27411d4

 ("xfs: make attr removal an explicit operation")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>