The function declarations in ext4.h are already marked extern, so it's
not necessary to do so in the .c files.

This quiets the sparse noise:

warning: function 'ext4_flush_completed_IO' with external linkage has definition
warning: function 'ext4_init_inode_table' with external linkage has definition
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>