[LU-15492] Fix fallthrough decorator to use __attribute__((fallthrough)) for gcc 7

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.16.0, Lustre 2.15.1
• Severity: 3
• Assignee: Shaun Tancheff
• Reporter: Shaun Tancheff

Description

Using the new fallthrough macro, building on openSUSE 15 with gcc-7.5

error: this statement may fall through [-Werror=implicit-fallthrough=]

Where the fallthrough attribute is available prefer it over a no-op suppression of
the implicit fallthrough warning.

Ex:

# if defined(__GNUC__) && __GNUC__ >= 7
#  define fallthrough  __attribute__((fallthrough)) /* fallthrough */
# else
#  define fallthrough do {} while (0)  /* fallthrough */
# endif