Fix compile error and warnings for old gcc versions

Fix compile error and warnings for old gcc versions

-Wimplicit-fallthrough option was added in gcc7 and
-Wpedantic was added in gcc4.8, while #pragma GCC diagnostic
support was not available until gcc4.6

We got following warnings:
../lib/ext2fs/fiemap.h:35: warning: expected [error|warning|ignored] after ‘#pragma GCC diagnostic’
../lib/ext2fs/fiemap.h:36: warning: unknown option after ‘#pragma GCC diagnostic’ kind
../lib/ext2fs/fiemap.h:38: warning: expected [error|warning|ignored] after ‘#pragma GCC diagnostic’

and error:
filefrag.c: In function ‘main’:
filefrag.c:577: error: #pragma GCC diagnostic not allowed inside functions
filefrag.c:578: error: #pragma GCC diagnostic not allowed inside functions
filefrag.c:595: error: #pragma GCC diagnostic not allowed inside functions

when compiling latest e2fsprogs with a gcc older than 4.6
e.g. on CentOS 6.9

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>