lib/mpi: use "static inline" instead of "extern inline"

When we use CONFIG_PROFILE_ALL_BRANCHES, every 'if()' introduces
a static variable, but that is not allowed in 'extern inline'
functions:

mpi-inline.h:116:204: warning: '______f' is static but declared in inline function 'mpihelp_sub' which is not static
mpi-inline.h:113:184: warning: '______f' is static but declared in inline function 'mpihelp_sub' which is not static
mpi-inline.h:70:184: warning: '______f' is static but declared in inline function 'mpihelp_add' which is not static
mpi-inline.h:56:204: warning: '______f' is static but declared in inline function 'mpihelp_add_1' which is not static

This changes the MPI code to use 'static inline' instead, to get
rid of hundreds of warnings.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>