ufs_free_fragments(): fix the braino in sanity check

ufs_free_fragments(): fix the braino in sanity check

The function expects that all fragments it's been asked to free will
be within the same block.  And it even has a sanity check verifying
that - it takes the fragment number modulo the number of fragments
per block, adds the count and checks if that's too high.

Unfortunately, it misspells the upper limit - instead of ->s_fpb
(fragments per block) it says ->s_fpg (fragments per cylinder group).
So "too high" ends up being insanely lenient.

Had been that way since 2.1.112, when UFS write support had been
added.  27 years to spot a typo...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>