remove misleading comment about f_frsize field

remove misleading comment about f_frsize field

It is supported both in the sense of being contained in mips version
of statfs/statfs64 structs and also in the sense that it's filled by
the corresponding kernel syscalls.

It is UNsupported in that sense that it's value is the same as that
of f_bsize (at least on older kernel versions, the oldest version
tested is 2.6.13), s. [1] and [2] for details ([1] is the latest
kernel version as of now, [2] is the oldest kernel version git
history is available for).

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/fs/statfs.c?h=v4.14-rc7#n64
[2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/fs/open.c?h=v2.6.12-rc2#n41

Follow-up of 68de9946e914d8c30dcc6667a059ea59e5b74cac

Signed-off-by: Eugene Rudoy <gene.devel@gmail.com>
Signed-off-by: Ralf Friedl <Ralf.Friedl@online.de>