ext4: Replace get_ext_path macro with an inline funciton

Replace get_ext_path macro with an inline function,
since this macro looks like a function call but its arguments
get modified. Ted pointed this out, thanks.

Signed-off-by: Akira Fujita <a-fujita@rs.jp.nec.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>