Since function ‘ext4_getfsmap_find_fixed_metadata’ can be made static,
make it so. Remove the following gcc warning (W=1):

  fs/ext4/fsmap.c:405:5: warning: no previous prototype for ‘ext4_getfsmap_find_fixed_metadata’ [-Wmissing-prototypes]
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>