xfs: Fix error code in 'xfs_ioc_getbmap()'

xfs: Fix error code in 'xfs_ioc_getbmap()'

In this function, once 'buf' has been allocated, we unconditionally
return 0.
However, 'error' is set to some error codes in several error handling
paths.
Before commit 957f4e6d932e ("xfs: simplify the xfs_getbmap interface")
this was not an issue because all error paths were returning directly,
but now that some cleanup at the end may be needed, we must propagate the
error code.

Fixes: 957f4e6d932e ("xfs: simplify the xfs_getbmap interface")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>