vfs_gpfs: update btime in vfs_gpfs_(f)get_dos_attributes

This paves the way for removing btime updates from the stat VFS
functions.

This way we behave like the default VFS module where DOS attributes and
btime are fetched from the same backing store and the frontend is
designed around using dos_mode() -> SMB_VFS_GET_ATTRIBUTES to update
both attributes as necessary in the SMB processing.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Christof Schmitt <cs@samba.org>
