Fix UBSAN if s_log_groups_per_flex is 31

Fix UBSAN if s_log_groups_per_flex is 31

It is logal (albeit rare) for the number of block groups per flex_bg
to 2**31 (which effectively means to put all of the block groups into
a single flex_bg).  However, in that case "1 << 31" is undefined on
architectures with a 32-bit integer.  Fix this UBSAN complaint by
using "1U << 31" instead.

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