ext2: include fs.h into ext2_fs.h

AC_CHECK_HEADERS([linux/ext2_fs.h])
fails with

configure:34666: checking linux/ext2_fs.h usability
configure:34666: gcc -std=gnu99 -c -ggdb3 -O0 -Wunreachable-code  conftest.c >&5
In file included from conftest.c:406:0:
/usr/include/linux/ext2_fs.h: In function 'ext2_mask_flags':
/usr/include/linux/ext2_fs.h:182:21: error: 'FS_DIRSYNC_FL' undeclared (first use in this function)
/usr/include/linux/ext2_fs.h:182:21: note: each undeclared identifier is reported only once for each function it appears in
/usr/include/linux/ext2_fs.h:182:37: error: 'FS_TOPDIR_FL' undeclared (first use in this function)
/usr/include/linux/ext2_fs.h:184:19: error: 'FS_NODUMP_FL' undeclared (first use in this function)
/usr/include/linux/ext2_fs.h:184:34: error: 'FS_NOATIME_FL' undeclared (first use in this function)

It's reasonable to have headers that include all necessary definitions. So fix
this by including fs.h into ext2_fs.h.

Signed-off-by: Petr Uzel <petr.uzel@suse.cz>
Signed-off-by: Jan Kara <jack@suse.cz>