xfs: clean up the XFS_IOC_{GS}ET_RESBLKS handler

The XFS_IOC_GET_RESBLKS and XFS_IOC_SET_RESBLKS already share a fair
amount of code, and will share even more soon.  Move the logic for both
of them out of the main xfs_file_ioctl function into a
xfs_ioctl_getset_resblocks helper to share the code and prepare for
additional changes.

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>