IB/uverbs: Fix reading of 32 bit flags

This is missing a zeroing of the high bits of flags, and is also not
correct for big endian machines. Properly zero extend the 32 bit flags
into the 64 bit stack variable.

Reported-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Fixes: bccd0622 ("IB/uverbs: Add UVERBS_ATTR_FLAGS_IN to the specs language")
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>