ext4: fix fortify warning in fs/ext4/fast_commit.c:1551

With the new fortify string system, rework the memcpy to avoid this
warning:

memcpy: detected field-spanning write (size 60) of single field "&raw_inode->i_generation" at fs/ext4/fast_commit.c:1551 (size 4)

Cc: stable@kernel.org
Fixes: 54d9469b

 ("fortify: Add run-time WARN for cross-field memcpy()")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>