fuse4fs: increase attribute timeout in iomap mode

In iomap mode, we trust the kernel to cache file attributes, because it
is critical to keep all of the file IO permissions checking in the
kernel as part of keeping all the file IO paths in the kernel.
Therefore, increase the attribute timeout to 30 seconds to reduce the
number of upcalls even further.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>