ext4: fixup 64-bit divides in 3.0-stable backport of upstream fix

Replace C division operators with div64_u64 for divides introduced in:
commit 503f4bd
ext4: use atomic64_t for the per-flexbg free_clusters count

Specific to the 3.0-stable backport of the upstream patch.

Change-Id: Id10180ed16f54f58fc18531092e88f52fc6de31c
Reviewed-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Todd Poynor <toddpoynor@google.com>