Fix a memory leak introduced in r328426.

[markj]

Summary

ffs_sbget() may return a superblock buffer even if it fails. Moreover,
readfunc returns a dynamically allocated buffer, when we call it on
backup superblocks, we need to free the previously allocated buffer.
Also fix what I believe is a free() of an uninitialized pointer if the
ffs_sbget() call in ffs_mountfs() fails before "fs" is set.

Test Plan

pho reported finding the leak while testing gmirror, tracked it down
to the aforementioned revision, and tested an earlier version of this
diff.

[kib]

lib/libufs/sblock.c

153

if () is redundant.

sys/ufs/ffs/ffs_subr.c

217

Most likely, this if () is redundant.

sys/ufs/ffs/ffs_vfsops.c

1078

I think if() is redundant.

[markj]

• Remove unneeded if statements.

[pho]

I ran tests on D14390.39361 for 7 hours. LGTM.