libext2fs: use statement-expression for container_of only on GNU-compatible compilers

libext2fs: use statement-expression for container_of only on GNU-compatible compilers

Functionally, the statement-expression is not necessary here; it
just gives a bit of type-safety to make sure the pointer really
does have a compatible type with the specified member of the struct.

When statement expressions are not available, we can just use a
portable fallback macro that skips this member type check.

Signed-off-by: Michael Forney <mforney@mforney.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>